[libc++] Deprecates and removes shared_ptr::unqiue. (#76576)

The status table incorrectly marks P0521R0 as nothing to do. This is not
correct the function should be deprecated.
During our latest monthly meeting we argreed to remove the
_LIBCPP_ENABLE_CXXyy_REMOVED_FEATURES macros, therefore the new macro is
not
added to that global list.

Implements
- P0521R0 Proposed Resolution for CA 14 (shared_ptr use_count/unique)

Implements parts of
- P0619R4 Reviewing Deprecated Facilities of C++17 for C++20

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Co-authored-by: Nikolas Klauser <nikolasklauser@berlin.de>