[libc++] Remove explicit instantiations of __insertion_sort_incomplete and __sort5 from the dylib

These instantiations were never visible, because they are only used in `__sort`, which is also explicitly instantiated in the dylib.

Reviewed By: ldionne, #libc

Spies: #libc_vendors, emaste, nemanjai, libcxx-commits

Differential Revision: https://reviews.llvm.org/D142185