CustomTarget: remove unnecessary dependency

There already is a dependency from the library to the package, so this
dependency should not be necessary.
Furthermore, the command is executed spuriously always, because the .done
file is always newer than the generated files.

solenv/gbuild/CustomTarget.mk

@@ -59,8 +59,6 @@ $(if $(filter $(2),$(gb_CustomTarget_REPOSITORYNAMES)),,\
 gb_CustomTarget_REPO_$(1) := $(2)
 $(call gb_CustomTarget_get_target,$(1)) : \
   $(call gb_CustomTarget__get_makefile,$($(2)),$(1))
-$(call gb_CustomTarget_get_workdir,$(1))/% : $(call gb_CustomTarget_get_target,$(1))
-	test -f $@ || (echo "Missing file $@ which should have been generated from CustomTarget $(1), but was not." && false)
 endef