doc: delete non-existing zlib constants

Checklist

• make -j4 test passes
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

Description of change

Fixes #7204 (closed).

Some constants in the zlib docs are not in the actual code:

zlib.Z_BINARY zlib.Z_TEXT zlib.Z_ASCII zlib.Z_UNKNOWN

Also handled in #7203, but that PR is marked as semver-major, so it will not land in v6.x.