[dev.typeparams] runtime: remove variadic defer/go calls

Now that defer/go wrapping is used, deferred/go'd functions are
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: I87bfd3fb2d759fbeb6487b8125c0f6992863d6e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/325915


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>