runtime: eliminate GOMAXPROCS limit

runtime: eliminate GOMAXPROCS limit

Now that allp is dynamically allocated, there's no need for a hard cap
on GOMAXPROCS.

Fixes #15131.

Change-Id: I53eee8e228a711a818f7ebce8d9fd915b3865eed
Reviewed-on: https://go-review.googlesource.com/45574
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>