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>