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

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

depends/funcs.mk

@@ -6,7 +6,6 @@ $(1)_objc=$$($$($(1)_type)_OBJC)
 $(1)_objcxx=$$($$($(1)_type)_OBJCXX)
 $(1)_ar=$$($$($(1)_type)_AR)
 $(1)_ranlib=$$($$($(1)_type)_RANLIB)
-$(1)_libtool=$$($$($(1)_type)_LIBTOOL)
 $(1)_nm=$$($$($(1)_type)_NM)
 $(1)_cflags=$$($$($(1)_type)_CFLAGS) \
             $$($$($(1)_type)_$$(release_type)_CFLAGS)