[dev.typeparams] cmd/compile: don't export blank functions in unified IR

After the previous two CLs, there's no need for unified IR to
write/read blank functions anymore: types2 has already checked that
they're valid, and the compiler backend is going to ignore them.

Allows dropping code for worrying about blank methods and will
probably simplify some of the object handling code eventually too.

Fixes #47446.

Change-Id: I03cb722793d676a246b1ab768b5cf0d3d2578b12
Reviewed-on: https://go-review.googlesource.com/c/go/+/338096


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>