Remove volatile qualifiers from pg_stat_statements.c.

Remove volatile qualifiers from pg_stat_statements.c.

Prior to commit 0709b7ee72, which changed the spinlock primitives
to function as compiler barriers, access to variables within a
spinlock-protected section required using a volatile pointer, but
that is no longer necessary.

Reviewed-by: Bertrand Drouvot, Michael Paquier
Discussion: https://postgr.es/m/Zqkv9iK7MkNS0KaN%40nathan