runtime: remove guard against uninitialized forcegc.g

The race was in the old C code.
The new Go code does not have the race
and does not need the check.

LGTM=bradfitz, dvyukov
R=golang-codereviews, bradfitz, dvyukov
CC=golang-codereviews, rlh
https://golang.org/cl/140180043
1 file changed