environment/pkg-config: unset PKG_CONFIG; find it via PATH instead.

... otherwise some pkgs fail to build due to host vs target (gtk+3).

common/environment/configure/pkg-config.sh

@@ -3,6 +3,3 @@
 if [ -z "$CHROOT_READY" ]; then
 	export PKG_CONFIG_PATH="${XBPS_MASTERDIR}/usr/lib/pkgconfig:${XBPS_MASTERDIR}/usr/share/pkgconfig"
 fi
-if [ -n "$CROSS_BUILD" ]; then
-	export PKG_CONFIG="${XBPS_CROSS_TRIPLET}-pkg-config"
-fi