Remove valgrind suppression for old zlib version

The fuzzbot has been upgraded, so hopefully this is no longer necessary.

Change-Id: Ia2d8ae03180c6e0b81a4f7b69b72610d047307ff
Reviewed-on: https://code.wireshark.org/review/3194
Reviewed-by: Evan Huus <eapache@gmail.com>