Add keep rule for keeping generic signature of return types (#3886)

* Add keep rule for keeping generic signature of return types

* Remove now obsolete shrinker rules

---------

Co-authored-by: Jake Wharton <jw@squareup.com>

retrofit/src/main/resources/META-INF/proguard/retrofit2.pro

@@ -35,11 +35,11 @@
 -if interface * { @retrofit2.http.* <methods>; }
 -keep,allowobfuscation interface * extends <1>
 
-# Keep generic signature of Call, Response (R8 full mode strips signatures from non-kept items).
--keep,allowobfuscation,allowshrinking interface retrofit2.Call
--keep,allowobfuscation,allowshrinking class retrofit2.Response
-
 # With R8 full mode generic signatures are stripped for classes that are not
 # kept. Suspend functions are wrapped in continuations where the type argument
 # is used.
 -keep,allowobfuscation,allowshrinking class kotlin.coroutines.Continuation
+
+# R8 full mode strips generic signatures from return types if not kept.
+-if interface * { @retrofit2.http.* public *** *(...); }
+-keep,allowoptimization,allowshrinking,allowobfuscation class <3>