pgsql: Remove some inappropriate #includes.

Remove some inappropriate #includes.

Other header files should never #include postgres.h (nor postgres_fe.h,
nor c.h), per project policy. Also, there's no need for any backend .c
file to explicitly include elog.h or palloc.h, because postgres.h pulls
those in already.

Extracted from a larger patch by Kyotaro Horiguchi. The rest of the
removals he suggests require more study, but these are no-brainers.

Discussion: https://postgr.es/m/20180215.200447.209320006.horiguchi.kyotaro@lab.ntt.co.jp

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/49bff412edd9eb226e146f6e4db7b5a8e843bd1f

Modified Files
--------------
src/backend/lib/knapsack.c | 1 -
src/backend/replication/basebackup.c | 1 -
src/backend/utils/misc/pg_config.c | 1 -
src/backend/utils/misc/rls.c | 1 -
src/include/lib/knapsack.h | 1 -
src/pl/plpython/plpy_spi.h | 1 -
src/pl/plpython/plpy_util.c | 1 -
7 files changed, 7 deletions(-)