pgsql: Remove char_tolower() API.

Remove char_tolower() API.

It's only useful for an ILIKE optimization for the libc provider using
a single-byte encoding and a non-C locale, but it creates significant
internal complexity.

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/450ceb6260cad30d7afdf155d991a9caafee7c0d.camel@j-davis.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1e493158d3d25771ed066028c00cbbdb41573496

Modified Files
--------------
src/backend/utils/adt/like.c           | 46 +++++++++++++---------------------
src/backend/utils/adt/like_match.c     | 18 +++++++------
src/backend/utils/adt/pg_locale.c      | 26 -------------------
src/backend/utils/adt/pg_locale_libc.c | 10 --------
src/include/utils/pg_locale.h          |  9 -------
5 files changed, 28 insertions(+), 81 deletions(-)