[dev.typeparams] cmd/compile: remove OCALLMETH Fatals in SSA generation

CL 330831 moved rewrite method calls to typecheck pass, then add Fatalf
check for mis-used of OCALLMETH in all frontend passes. The check in SSA
generation pass is now redundant.

Change-Id: If959b774ab3c3aeefabf32333f77b9a93c156ce3
Reviewed-on: https://go-review.googlesource.com/c/go/+/330834


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>