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

"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
1 file changed