[Xamarin.Android.Tools.AndroidSdk] Fix a few nullability warnings (#97)

Fixes a few Nullable Reference Type (NRT) warnings in
`JdkInfoVersionComparer`.
There are also some NRT warnings in `EqualityComparer<T>`, but this
class is `private` and not used anywhere; remove it instead.

Author: jpobst

src/Xamarin.Android.Tools.AndroidSdk/AndroidVersions.cs

@@ -184,26 +184,4 @@ static bool MatchesId (AndroidVersion version, string id)
 },
 };
 }
-
-classEqualityComparer<T>:IEqualityComparer<T>
-{
-Func<T,T,bool>equals;
-Func<T,int>getHashCode;
-
-publicEqualityComparer(Func<T,T,bool>equals,Func<T,int>?getHashCode=null)
-{
-this.equals=equals;
-this.getHashCode=getHashCode??(v =>v?.GetHashCode()??0);
-}
-
-publicboolEquals(Tx,Ty)
-{
-returnequals(x,y);
-}
-
-publicintGetHashCode(Tobj)
-{
-returngetHashCode(obj);
-}
-}
 }

src/Xamarin.Android.Tools.AndroidSdk/JdkInfo.cs

@@ -492,9 +492,9 @@ class JdkInfoVersionComparer : IComparer<JdkInfo>
 {
 publicstaticreadonlyIComparer<JdkInfo>Default=newJdkInfoVersionComparer();
 
-publicintCompare(JdkInfox,JdkInfoy)
+publicintCompare([AllowNull]JdkInfox,[AllowNull]JdkInfoy)
 {
-if(x.Version!=null&&y.Version!=null)
+if(x?.Version!=null&&y?.Version!=null)
 returnx.Version.CompareTo(y.Version);
 return0;
 }