[libc++] [test] Eliminate the libcpp-no-if-constexpr feature flag.

At this point, every supported compiler that claims a -std=c++17 mode
should also support `if constexpr`. This was an issue for GCC 5
and GCC 6, but hasn't been an issue since GCC 7. (Our current
minimum supported GCC version, IIUC, is GCC 10 or 11.)

Differential Revision: https://reviews.llvm.org/D113348