cmd/compile: prune some deadcode around ODCLFUNC

cmd/compile: prune some deadcode around ODCLFUNC

An *ir.Func is always ODCLFUNC, so no need to double-check this
anymore. The type system statically ensures we have the right Op.

Also, pkginit.initRequiredForCoverage appears to be unused, so we can
get rid of it completely.

Change-Id: If1abb35672b40f705f23c365ad2a828c2661e9c0
Reviewed-on: https://go-review.googlesource.com/c/go/+/520603
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>