cmd/compile: eliminate fallback code for missing -p flag

cmd/compile: eliminate fallback code for missing -p flag

cmd/compile has required the -p flag since go.dev/cl/391014. It's safe
to eliminate the fallback code that tried to cope without.

Change-Id: I9a62ff829e34a6fa5bfe6ae6a836610cc3f0cd33
Reviewed-on: https://go-review.googlesource.com/c/go/+/523337
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>