pgsql: Fix some comments related to library unloading

Fix some comments related to library unloading

Library unloading has never been supported with its code removed in
ab02d702ef08, and there were some comments still mentioning that it was
a possible operation.

ChangAo has noticed the incorrect references in dfmgr.c, while I have
noticed the other ones while scanning the rest of the tree for similar
mistakes.

Author: ChangAo Chen, Michael Paquier
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/tencent_1D09840A1632D406A610C8C4E2491D74DB0A@qq.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/7f97b4734f937db6f8dab1bbf8bbaab349e6c9b1

Modified Files
--------------
contrib/auto_explain/auto_explain.c             | 2 +-
contrib/passwordcheck/passwordcheck.c           | 2 +-
contrib/pg_stat_statements/pg_stat_statements.c | 2 +-
contrib/sepgsql/hooks.c                         | 2 +-
src/backend/utils/fmgr/dfmgr.c                  | 4 ++--
5 files changed, 6 insertions(+), 6 deletions(-)