cmd/compile: remove PDISCARD class

This pseudo-Class was previously used by the importer code to optimize
processing duplicate inline bodies, since we didn't actually care
about the declarations in those contexts.

This functionality is no longer needed with indexed exports, since we
now only process function bodies as needed (and never more than once).

Passes toolstash-check.

Change-Id: I7eab0cb16925ab777079c2a20731dbcfd63cf195
Reviewed-on: https://go-review.googlesource.com/c/go/+/200899


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>