[One .NET] Do not preserve GeneratedEnumAttribute (#5450)

Context: #5167

The `GeneratedEnumAttribute` attribute does not need to be preserved
for runtime behavior.  It is only used for bindings.

Removing the attribute allows apps to be smaller.

`.apk` size comparison, BuildReleaseArm64False test:

	> apkdiff -f -e dll$ before.apk after.apk
	Size difference in bytes ([*1] apk1 only, [*2] apk2 only):
	  -          39 assemblies/Mono.Android.dll
	    +         132 Resource Android.ILLink.ILLink.LinkAttributes.xml
	    -             Type Android.Runtime.GeneratedEnumAttribute
	Summary:
	  -          39 Assemblies -0.01% (of 749,078)

Note that the attribute instances removal doesn't work yet; we need
net6 illink for that.

src/Microsoft.Android.Sdk.ILLink/PreserveLists/Mono.Android.xml

@@ -3,7 +3,6 @@
 	<assembly fullname="Mono.Android">
 		<type fullname="Android.Runtime.AnnotationAttribute" />
 		<type fullname="Android.Runtime.GeneratedDummyHost" />
-		<type fullname="Android.Runtime.GeneratedEnumAttribute" />
 		<type fullname="Android.Runtime.IJavaObject" />
 		<type fullname="Android.Runtime.JNIEnv">
 			<method name="Exit" />

src/Mono.Android/ILLink/ILLink.LinkAttributes.xml

@@ -6,5 +6,8 @@
     <type fullname="Android.Runtime.IntDefinitionAttribute">
       <attribute internal="RemoveAttributeInstances" />
     </type>
+    <type fullname="Android.Runtime.GeneratedEnumAttribute">
+      <attribute internal="RemoveAttributeInstances" />
+    </type>
   </assembly>
 </linker>