Remove volatile qualifiers from shm_mq.c.
Since commit
0709b7ee, spinlock primitives include a compiler barrier
so it is no longer necessary to access either spinlocks or the memory
they protect through pointer-to-volatile. Like earlier commits
e93b6298,
d53e3d5f,
430008b5,
8f6bb851,
df4077cd.
Author: Thomas Munro
Discussion: https://postgr.es/m/CAEepm=204T37SxcHo4=xw5btho9jQ-=ZYYrVdcKyz82XYzMoqg@mail.gmail.com