package/mupdf: drop MUPDF_PKG_CONFIG_PACKAGES
This variable might have been needed in the very distant past to help
mupdf find libraries, but mupdf's Makefile now properly uses
pkg-config to find the 5 libraries freetype2, gumbo, harfbuzz, libjpeg
and zlib, so our code has become redundant: drop it.
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>