cmd/compile/internal/pgo/internal/graph: delete dead code

cmd/compile/internal/pgo/internal/graph: delete dead code

We don't use large swaths of this package. Delete the code. We can
always bring it back later if needed.

Change-Id: I6b39a73ed9c48d2d5b37c14763d7bb7956f3ef43
Reviewed-on: https://go-review.googlesource.com/c/go/+/494438
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>