Remove unused code in the interop layers

[filipnavara]

Casual look at the Java.Interop/java-interop projects that there's a lot of code that can be removed since it's no longer used:

• Any code that supports Java's WeakReference instead of the JNI weak handle. This option is not used anymore but the native code to support it still exists:

  java-interop/src/java-interop/java-interop-gc-bridge-mono.cc

  Lines 1288 to 1292 in 5bd93ae

  	case JAVA_INTEROP_GC_BRIDGE_USE_WEAK_REFERENCE_KIND_JAVA:
  	message = "Using java.lang.ref.WeakReference for JNI Weak References.";
  	take_global_ref = take_global_ref_java;
  	take_weak_global_ref = take_weak_global_ref_java;
  	break;

• java_interop_strdup / java_interop_free

[jonpryor]

I had written:

The WeakReference code is effectively dead. The contents of src/java-interop is a "desktop port" of GC bridge code from dotnet/android for use with MonoVM, which we never got around to unifying. The WeakReference backend is no longer supported; see dotnet/android@2f93f76

Additionally, note that the use of WeakReference was when JNI Weak Global References didn't work (Android < API-14).

I was wrong. (Or at least, not quite right.)

dotnet/android@2f93f763 removes support for JNI Global References being direct object pointers. Meaning instead of there being an indirection (GREF value > GREF table > object instance), the GREF value was the object instance. In a world in which objects can move around in memory due to GC, "direct" references cannot work, but Android worked this way for quite some time (through API-13).

Consequently, dotnet/android@2f93f763 is entirely unrelated to our code to use java.lang.ref.WeakReference instead of JNI Weak Global References.

The java.lang.ref.WeakReference code can be used by setting the debug.mono.wref system property.

That said, we have no current tests which set this property, and I see no obvious usage of debug.mono.wref on GitHub, so while this code may not be "dead", it's close to it

[jonpryor]

dotnet/android@e98ae9c removes support for debug.mono.wref=java, java.lang.ref.WeakReference, and the weak_handle fields from Java.Lang.Object, Java.Lang.Exception, Java.Interop.JavaObject, and Java.Interop.JavaException.

Development of #1257 can resume, if desired.

[filipnavara]

Development of #1257 can resume, if desired.

I'll get to it in the upcoming week[s]. Thanks for the Android PR!