depends: remove (darwin) libtool now that it's no longer used

Note that this is completely unrelated to gnu usage of libtool.

depends/packages.md

@@ -74,7 +74,6 @@ These variables may be set to override or append their default values.
     $(package)_objcxx
     $(package)_ar
     $(package)_ranlib
-    $(package)_libtool
     $(package)_nm
     $(package)_cflags
     $(package)_cxxflags