pgsql: Simplify PGAC_STRUCT_TIMEZONE Autoconf macro

Simplify PGAC_STRUCT_TIMEZONE Autoconf macro

Since 63bd0db12199c5df043e1dea0f2b574f622b3a4c we don't use tzname
anymore, so we don't need to check for it.  Instead, just keep the
part of PGAC_STRUCT_TIMEZONE that we need, which is the check for
struct tm.tm_zone.

Discussion: https://www.postgresql.org/message-id/flat/5eb11a37-f3ca-5fb7-308f-4485dec25a2e%402ndquadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/400d5ffcafa65e0f742dd29de83b035b8ea27c4a

Modified Files
--------------
config/c-library.m4           | 31 ++++-------------
configure                     | 78 +------------------------------------------
src/include/pg_config.h.in    | 10 ------
src/include/pg_config.h.win32 | 10 ------
src/pl/plpython/plpython.h    |  1 -
5 files changed, 7 insertions(+), 123 deletions(-)