runtime/pprof: fix data race
It's unclear why we do this broken double-checked locking.
The mutex is not held for the whole duration of CPU profiling.
Fixes issue 8365.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/116290043
runtime: remove FlagNoProfile
Turns out to be unused as well.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews, khr
https://codereview.appspot.com/127170044