bpo-40686: Fix compiler warnings on _zoneinfo.c (GH-23614) (GH-23804)

bpo-40686: Fix compiler warnings on _zoneinfo.c (GH-23614) (GH-23804)

"uint8_t day" is unsigned and so "day < 0" test is always true.
Remove the test to fix the following warnings on Windows:

modules\_zoneinfo.c(1224): warning C4068: unknown pragma
modules\_zoneinfo.c(1225): warning C4068: unknown pragma
modules\_zoneinfo.c(1227): warning C4068: unknown pragma
(cherry picked from commit aefb69b23f056c61e82ad228d950f348de090c70)

Co-authored-by: Victor Stinner <vstinner@python.org>
Co-authored-by: Victor Stinner <vstinner@python.org>