Remove redundant type arguments

In Java 11 type arguments for anonymous subclasses can be inferred
and don't need to be specified. This resolves a number of compiler
warnings.

This is Java 8 incompatible change, but that's fine because Gerrit core
also removed redundant type arguments (see change If9ebda63f).

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: I5602f5028024606f279366b4d9ec4db698927996