Bug 1590907 - Remove obsolete macro MOZ_HAVE_BITSCAN64. r=froydnj

This macro seems to have been introduced in
https://bugzilla.mozilla.org/attachment.cgi?id=772469&action=diff
but there doesn't seem to be any mention of it anywhere else. Currently
it's only checked in order to #undef it if it's defined.

Differential Revision: https://phabricator.services.mozilla.com/D51770

--HG--
extra : moz-landing-system : lando

mfbt/MathAlgorithms.h

@@ -245,10 +245,6 @@ inline uint_fast8_t CountTrailingZeroes64(uint64_t aValue) {
 #  endif
 }
 
-#  ifdef MOZ_HAVE_BITSCAN64
-#    undef MOZ_HAVE_BITSCAN64
-#  endif
-
 #elif defined(__clang__) || defined(__GNUC__)
 
 #  if defined(__clang__)