Removed superfluous #if's in predef/other/endian.h

There were some superfluous #if's in endian.h:

#if !BOOST_ENDIAN_BIG_BYTE && !BOOST_ENDIAN_BIG_WORD && \
    !BOOST_ENDIAN_LITTLE_BYTE && !BOOST_ENDIAN_LITTLE_WORD
#   if !BOOST_ENDIAN_BIG_BYTE

I removed them.