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: /messages/by-id/a0f817ee-fb86-483a-8a14-b6f7f5991b6e@eisentraut.org

Branch
------
master

Details
-------
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(-)