Remove a now-obsolete microoptimization for a few old Japanese character sets
(bswap is single-cycle these days, not to mention movbe), allowing us to remove
the link from m_ctype.h to my_byteorder.h. This is relevant because m_ctype.h
is included from a huge amount of places, and my_byteorder.h pulls in quite a
few lines transitively. (We remove one of the worst offenders, though, which was
mistakenly added in an earlier IWYU run.)

Change-Id: I49e2f2de371b9dd7da78a6692992dd9e9bd28e8f