Fix nuspec after .NET standard rollback

nuget/MacMobileCenter.nuspec

@@ -13,9 +13,6 @@
     <licenseUrl>https://aka.ms/vbgfx2</licenseUrl>
     <iconUrl>https://aka.ms/xhh7sr</iconUrl>
     <dependencies>
-        <group targetFramework="netstandard1.0">
-            <dependency id="NETStandard.Library" version="1.6.1" />
-        </group>
         <!-- Use dependency groups for iOS and Android so that additional dependencies aren't added to them -->
         <group targetFramework="MonoAndroid403">
         </group>

nuget/MobileCenter.nuspec

@@ -13,9 +13,6 @@
     <licenseUrl>https://aka.ms/vbgfx2</licenseUrl>
     <iconUrl>https://aka.ms/xhh7sr</iconUrl>
     <dependencies>
-      <group targetFramework="netstandard1.0">
-          <dependency id="NETStandard.Library" version="1.6.1" />
-      </group>
       <group targetFramework="uap10.0">
         <dependency id="Microsoft.NETCore.UniversalWindowsPlatform" version="5.2.2"/>
         <dependency id="Newtonsoft.Json" version="6.0.1"/>