[dev.typeparams] cmd/compile: remove variadic defer calls

Now that defer wrapping is used, deferred function is always
argumentless. Remove the code handling arguments.

This CL is mostly removing the fallback code path. There are more
cleanups to be done, in later CLs.

Change-Id: If6c729d3055c7a507cb1f1a000f5bbd3ad7ff235
Reviewed-on: https://go-review.googlesource.com/c/go/+/325914


Trust: Cherry Mui <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>