gbuild: remove unused gb_LinkTarget_use_headers

Change-Id: Idce74274169970bdd35869578cc0046fe502e614

solenv/gbuild/Library.mk

@@ -252,7 +252,6 @@ $(eval $(foreach method,\
 	use_package \
 	use_packages \
 	use_unpacked \
-	use_headers \
 	add_package_headers \
 	add_sdi_headers \
 	set_precompiled_header \

solenv/gbuild/LinkTarget.mk

@@ -1317,18 +1317,6 @@ $(call gb_LinkTarget_get_headers_target,$(1)) :| $(call gb_ExternalProject_get_t
 
 endef
 
-# Use headers provided by another link target.
-#
-# This function should only be needed to enforce correct build ordering,
-# if the link target does not use the other link target.
-#
-# gb_LinkTarget_use_headers linktarget other-linktarget(s)
-define gb_LinkTarget_use_headers
-$(call gb_LinkTarget_get_headers_target,$(1)) :\
-    $(foreach linktarget,$(2),$(call gb_LinkTarget_get_headers_target,$(linktarget)))
-
-endef
-
 # this forwards to functions that must be defined in RepositoryExternal.mk.
 # $(eval $(call gb_LinkTarget_use_external,library,external))
 define gb_LinkTarget_use_external