pgsql: Remove support for null pg_locale_t most places.

Remove support for null pg_locale_t most places.

Previously, passing NULL for pg_locale_t meant "use the libc provider
and the server environment". Now that the database collation is
represented as a proper pg_locale_t (not dependent on setlocale()),
remove special cases for NULL.

Leave wchar2char() and char2wchar() unchanged for now, because the
callers don't always have a libc-based pg_locale_t available.

Discussion: https://postgr.es/m/cfd9eb85-c52a-4ec9-a90e-a5e4de56e57d@eisentraut.org
Reviewed-by: Peter Eisentraut, Andreas Karlsson

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e9931bfb7515b253cc26ff495ee917acff8995d0

Modified Files
--------------
src/backend/access/hash/hashfunc.c | 10 ++--
src/backend/regex/regc_pg_locale.c | 113 ++-----------------------------------
src/backend/utils/adt/formatting.c | 84 +++++++--------------------
src/backend/utils/adt/like.c | 10 +---
src/backend/utils/adt/pg_locale.c | 73 +++++++++---------------
src/backend/utils/adt/varchar.c | 10 ++--
src/backend/utils/adt/varlena.c | 28 +++------
7 files changed, 73 insertions(+), 255 deletions(-)