pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8928817769de0d81758bc760333d3056c67b63c1

Modified Files
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contrib/pg_stat_statements/pg_stat_statements.c | 229 ++++++++++--------------
1 file changed, 95 insertions(+), 134 deletions(-)