cmd/compile/internal/types: remove Funarg

cmd/compile/internal/types: remove Funarg

There's no need for the Funarg type anymore. A simple boolean suffices
to indicate whether a TSTRUCT represents a parameter tuple.

While here, rename Struct.Funarg to ParamTuple.

Change-Id: I657512d4ba10e51ec4cfd7c7d77e0194bdb0853b
Reviewed-on: https://go-review.googlesource.com/c/go/+/521315
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>