[Driver] Remove obsoleted -gz=zlib-gnu

GCC added -gz=zlib-gnu in 2014 for -gz meaning change (.zdebug =>
SHF_COMPRESSED) and the legacy zlib-gnu hasn't gain adoption.

According to Debian Code Search (`gz=zlib-gnu`), no project uses -gz=zlib-gnu
(valgrind has a configure to use -gz=zlib). Any possible -gz=zlib-gnu user can
switch to -gz smoothly (supported by integrated assemblers for many years;
binutils 2.26).

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D117744