Remove libXp dependency from imake/motif ports

Remove the libXp dependency from a number of ports using imake and/or motif.
libXp is being deprecated.
The list of ports:
audio/xmcd
audio/xmmix
deskutils/xmdiary
ftp/llnlxdir
ftp/llnlxftp
graphics/SciPlot
misc/xtar
multimedia/mpeg2play