[catalyst] remove splash screens (#11300)

Context: https://github.com/dotnet/maui/pull/807

When implementing a new `@(MauiSplashScreen)` feature for .NET MAUI, I
found out that Catalyst doesn't have support for splash screens.

I even tested this in Xcode:

* Create a new Objective-C iOS app
* Make `LaunchScreen.storyboard` red and `Main.storyboard` green.
* Check the new `macOS` checkbox in project settings to enable
  Catalyst and be a multiplatform app.

I see red, then green on iOS but only green on macOS. I think Catalyst
completely ignores `UILaunchStoryboardName`.

This removes splash screens from the `dotnet new maccatalyst` template
as well as the `MyCatalystApp` sample project.

dotnet/Microsoft.MacCatalyst.Templates/maccatalyst/Info.plist

@@ -16,8 +16,6 @@
     <array>
         <integer>2</integer>
     </array>
-    <key>UILaunchStoryboardName</key>
-    <string>LaunchScreen</string>
     <key>UISupportedInterfaceOrientations</key>
     <array>
         <string>UIInterfaceOrientationPortrait</string>

dotnet/Microsoft.MacCatalyst.Templates/maccatalyst/LaunchScreen.storyboard

@@ -1,26 +0,0 @@
-<?xml version="1.0" encoding="UTF-8" standalone="no"?>
-<document type="com.apple.InterfaceBuilder3.CocoaTouch.Storyboard.XIB" version="3.0" toolsVersion="13771" targetRuntime="iOS.CocoaTouch" propertyAccessControl="none" useAutolayout="YES" useTraitCollections="YES" colorMatched="YES" initialViewController="222" launchScreen="YES">
-    <dependencies>
-        <plugIn identifier="com.apple.InterfaceBuilder.IBCocoaTouchPlugin" version="13772"/>
-        <capability name="documents saved in the Xcode 8 format" minToolsVersion="8.0"/>
-    </dependencies>
-    <scenes>
-        <scene sceneID="221">
-            <objects>
-                <viewController id="222" sceneMemberID="viewController">
-                    <layoutGuides>
-                        <viewControllerLayoutGuide type="top" id="219"/>
-                        <viewControllerLayoutGuide type="bottom" id="220"/>
-                    </layoutGuides>
-                    <view key="view" contentMode="scaleToFill" id="223">
-                        <rect key="frame" x="0.0" y="0.0" width="414" height="736"/>
-                        <autoresizingMask key="autoresizingMask" widthSizable="YES" heightSizable="YES"/>
-                        <color key="backgroundColor" white="1" alpha="1" colorSpace="calibratedWhite"/>
-                    </view>
-                </viewController>
-                <placeholder placeholderIdentifier="IBFirstResponder" id="224" userLabel="First Responder" sceneMemberID="firstResponder"/>
-            </objects>
-            <point key="canvasLocation" x="-200" y="-388"/>
-        </scene>
-    </scenes>
-</document>

tests/dotnet/MyCatalystApp/Info.plist

@@ -18,7 +18,5 @@
 	<string>10.15</string>
 	<key>XSAppIconAssets</key>
 	<string>Resources/Images.xcassets/AppIcons.appiconset</string>
-	<key>XSLaunchImageAssets</key>
-	<string>Resources/Images.xcassets/LaunchImage.launchimage</string>
 </dict>
 </plist>

tests/dotnet/MyCatalystApp/Resources/Images.xcassets/LaunchImage.launchimage/Contents.json

@@ -1,33 +0,0 @@
-{
-  "images": [
-    {
-      "orientation": "portrait",
-      "idiom": "iphone",
-      "extent": "full-screen",
-      "filename": "Default.png",
-      "size": "320x480",
-      "scale": "1x"
-    },
-    {
-      "orientation": "portrait",
-      "idiom": "iphone",
-      "extent": "full-screen",
-      "filename": "Default@2x.png",
-      "size": "320x480",
-      "scale": "2x"
-    },
-    {
-      "orientation": "portrait",
-      "idiom": "iphone",
-      "extent": "full-screen",
-      "filename": "Default-568h@2x.png",
-      "size": "320x568",
-      "subtype": "retina4",
-      "scale": "2x"
-    }
-  ],
-  "info": {
-    "version": 1,
-    "author": "xcode"
-  }
-}