gdb/make-target-delegates.py: don't handle "void" in parse_argtypes

I suppose this was needed when we had `void` in declarations of methods
with no parameters.  If so, we no longer need it.  There are no changes
in the generated file.

Change-Id: I0a2b398408aa129634e2d73097a038f7f80db4b4
Approved-By: John Baldwin <jhb@FreeBSD.org>