[libc++] Revert LWG3223 Broken requirements for shared_ptr converting constructors (#93071)

This reverts commit d868f0970, which was shown to break some code and we
don't know yet whether the code should be valid or not. Reverting until
we've had time to figure it out next week.

NOKEYCHECK=True
GitOrigin-RevId: e7d563501f056d0ae54440468df04b04c10f8070
7 files changed