[Java.Interop-MonoAndroid] Remove NullableAttributes.cs to prevent duplicate conflict warnings.

[jpobst]

When building Java.Interop-MonoAndroid.csproj we currently get ~100 warnings of the form:

Java.Interop\JavaArray.cs(31,4): warning CS0436: The type 'MaybeNullAttribute' in 
'C:\A\vs2019xam00000Z-1\_work\1\s\external\Java.Interop\src\Java.Interop\NullableAttributes.cs' 
conflicts with the imported type 'MaybeNullAttribute' in 'mscorlib, Version=2.0.5.0, Culture=neutral, 
PublicKeyToken=7cec85d7bea7798e'. Using the type defined in 'C:\A\vs2019xam00000Z-1\_work
\1\s\external\Java.Interop\src\Java.Interop\NullableAttributes.cs'. [C:\A\vs2019xam00000Z-1\_work
\1\s\external\Java.Interop\src\Java.Interop\Java.Interop-MonoAndroid.csproj]

Exclude Java.Interop\src\Java.Interop\NullableAttributes.cs from our source files to eliminate the warnings.