[libc++] Disable tree invariant check in asserts mode

This is a follow-up to D153672 which removed the old debug mode and
moved many of those checks to the regular asserts mode.

The tree invariant check is too expensive for the regular asserts mode,
making element removal O(n) instead of O(log n), so disable it until
there is a new debug assert category it can be put in.

Differential revision: https://reviews.llvm.org/D154417