[NFC] Remove constexpr_icmp.ll test after (#2200)

It's still possible to rewrite it, but it (as well as the problem it
addresses) will become obsolete when
https://discourse.llvm.org/t/rfc-remove-most-constant-expressions/63179
is fully implemented.

Signed-off-by: Sidorov, Dmitry <dmitry.sidorov@intel.com>