internal/profile: remove legacy profile support

internal/profile: remove legacy profile support

internal/profile.Parse is only used in two places: cmd/compile for
parsing PGO profiles, and net/http/pprof for parsing runtime/pprof
profiles for delta profiles. Neither case ever encounters legacy
profiles, so we can remove support entirely from the package.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest

Change-Id: Ic5f85b3fc1e1367131b6039dac9378913cbf9f2c
Reviewed-on: https://go-review.googlesource.com/c/go/+/548035
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>