libstdc++: Optimize removal from unique assoc containers [PR112934]

libstdc++: Optimize removal from unique assoc containers [PR112934]

Previously, calling erase(key) on both std::map and std::set
would execute that same code that std::multi{map,set} would.
However, doing that is unnecessary because std::{map,set}
guarantee that all elements are unique.

It is reasonable to expect that erase(key) is equivalent
or better than:

  auto it = m.find(key);
  if (it != m.end())
    m.erase(it);

However, this was not the case. Fix that by adding a new
function _Rb_tree<>::_M_erase_unique() that is essentially
equivalent to the above snippet, and use this from both
std::map and std::set.

libstdc++-v3/ChangeLog:

	PR libstdc++/112934
	* include/bits/stl_map.h (map::erase): Use _M_erase_unique.
	* include/bits/stl_set.h (set::erase): Likewise.
	* include/bits/stl_tree.h (_Rb_tree::_M_erase_unique): Add.