chore: remove Kotlin version from the default template (#28626)

Summary:
The default application requires Kotlin version that is not supported by the Gradle plugin (should be at least `1.3.10`). However, instead of upgrading, we should remove it entirely. Here's why.

This commit 29d3dfbd19 introduced Detox for RNTester Android application.

Since the commit doesn't mention Detox for the default application and there are no Detox tests present by default in the default application, I believe that this addition was performed by a mistake.

The best way is to remove Kotlin from the default template. This step is described in the Detox documentation and all users that are integrating Detox will be asked to perform it anyway. No need to do it for them.

## Changelog

[ANDROID] [INTERNAL] - remove Kotlin from the default template
Pull Request resolved: https://github.com/facebook/react-native/pull/28626

Test Plan: Building a brand new project with `master` should work

Differential Revision: D21388961

Pulled By: shergin

fbshipit-source-id: 92666aa67f92b29f4e7f9c036b332bd058cdd49e

template/android/build.gradle

@@ -11,11 +11,8 @@ buildscript {
         google()
         jcenter()
     }
-    ext.kotlinVersion = '1.3.0'
     dependencies {
         classpath("com.android.tools.build:gradle:3.5.3")
-        // for Detox
-        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion"
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files
     }