pgsql: Remove checks for no longer supported GCC versions

Remove checks for no longer supported GCC versions

Since commit f5e0186f865 (Raise C requirement to C11), we effectively
require at least GCC version 4.7, so checks for older versions can be
removed.

Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/a0f817ee-fb86-483a-8a14-b6f7f5991b6e%40eisentraut.org

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/4fbe0151455fefbef7abc9d507adb04c978beb0d

Modified Files
--------------
src/include/c.h                        | 8 ++++----
src/include/port/atomics/generic-gcc.h | 4 ++--
2 files changed, 6 insertions(+), 6 deletions(-)