configure: Remove some redundant AC_SUBST usage
configure: Remove some redundant AC_SUBST usage

PKG_CHECK_MODULES has called these automatically since well before
pkg-config 0.24.

flatpak/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,amigadave/flatpak,amigadave/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,amigadave/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,amigadave/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,flatpak/flatpak,flatpak/flatpak