README: Remove mention of libXfont 1.5
xfs was ported to libXfont2 in release 1.2, and bdftopcf 1.1 includes a copy of enough of the old libXfont1 code to not need an external libXfont at all.
xfs was ported to libXfont2 in release 1.2, and bdftopcf 1.1 includes a copy of enough of the old libXfont1 code to not need an external libXfont at all.