lib/libz: remove ZLIBprivate_1.0 namespace.

These symbols were hidden and no longer exposed, and were never exposed in headers.

MFC after: 3 days

(cherry picked from commit a7f127b653c939976ac492595a8283b5de08ed0d)
(cherry picked from commit c4727a47f18cb54ebbab951157e57b019db09fc0)

Approved by: re (gjb)