Revert "Include build date in XeTeX banner"

This reverts commit f6d0f6ef27147c9179f343b67881155a67fd9e20.
Drop Mac-specific image loading code

We dropped support for xdv2pdf, so we can only support images supported
by xdvipdfmx now. There is no much point in using the Mac-specific code
and using the same code in all platforms is always better.
Some rearrangements
!XETEX_MAC is more clear than XETEX_OTHER