[dev.typeparams] cmd/compile: catch another mis-used OCALLMETH in backend

OCALLMETH is rewritten by walkCall to OCALLFUNC, and other places in
backend have already caught it. So do the same thing in state.expr for
consistency and prevent mis-use in frontend side.

While at it, also remove un-used function getParam.

Change-Id: I03e1ea907e0bcb05fa35fa81804c33b5c9a4d77e
Reviewed-on: https://go-review.googlesource.com/c/go/+/330669


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>