kbuild: remove unneeded dependency for $(DOC_TARGETS)

Commit 415008af ("docs-rst: convert lsm from DocBook to ReST")
stopped using if_changed_rule. There is no more users of if_changed*
for the doc targets. Hence, fixdep is unneeded. Remove the dependency
on scripts_basic.

All the doc targets are phony. The dependency on FORCE is not needed
either.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1575,7 +1575,7 @@ $(help-board-dirs): help-%:
 DOC_TARGETS := xmldocs latexdocs pdfdocs htmldocs epubdocs cleandocs \
 	       linkcheckdocs dochelp refcheckdocs
 PHONY += $(DOC_TARGETS)
-$(DOC_TARGETS): scripts_basic FORCE
+$(DOC_TARGETS):
 	$(Q)$(MAKE) $(build)=Documentation $@
 
 # Misc