[libc++] Remove __is_replaceable emulation (#167355)
The Trivial Relocation feature has been removed from the C++26 working
draft. Based on discussions in Kona, it is unlikely that the
"replaceable" type concept will come back in the C++29 time frame.
Since we don't have a use for the type trait in the library at the
moment, remove the code associated to it. If we end up needing something
like it in the future, we can always add it back.
