BuildSystem: Automatically delete libtool files

Half of the ports had a manual post_install step to delete libtool files
this is now done automatically and there is no need for it!

Libtool files have to be deleted as libtool doesn't work while
cross-compiling (at least out of the box)

ports/mpfr/build.sh

@@ -9,8 +9,3 @@ CONFIGURE_OPTIONS=(
 	"--target=$BANAN_TOOLCHAIN_TRIPLE"
 	'--with-sysroot=/'
 )
-
-post_install() {
-	# remove libtool file
-	rm -f $BANAN_SYSROOT/usr/lib/libmpfr.la
-}