CPack: Remove undocumented deprecated OSXX11 generator

This CPack generator was never documented, and has been deprecated since
commit 2ed00e8e (CPack: Deprecate OSXX11 generator, 2020-01-31,
v3.17.0-rc1~45^2).

Issue: #20235
40 jobs for !6704 with cpack-drop-osxx11 in 28 minutes and 39 seconds (queued for 2 seconds)
latest merge request