Drop dead glibc feature-macro juggling from gzguts.h

Drop dead glibc feature-macro juggling from gzguts.h

_LARGEFILE_SOURCE enables fseeko/ftello, which zlib-ng never calls
(gzlib.c uses lseek/lseek64/_lseeki64 directly). The _FILE_OFFSET_BITS
and _TIME_BITS undefs were defensive against a consumer-provided
-D_FILE_OFFSET_BITS=64 leaking into library internals, but they sat
after zbuild.h had already included <stdio.h> so they never affected
system-header sizing. The zlib.h gzopen->gzopen64 remap they were
guarding against is already blocked by the Z_INTERNAL gate for
library builds.