docs: Makefile: remove no-ops targets

After removal of DocBook, those targets are bogus.
Reported-by: NJim Davis <jim.epost@gmail.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/Makefile

@@ -95,16 +95,6 @@ endif # HAVE_SPHINX
 # The following targets are independent of HAVE_SPHINX, and the rules should
 # work or silently pass without Sphinx.
 
-# no-ops for the Sphinx toolchain
-sgmldocs:
-	@:
-psdocs:
-	@:
-mandocs:
-	@:
-installmandocs:
-	@:
-
 cleandocs:
 	$(Q)rm -rf $(BUILDDIR)
 	$(Q)$(MAKE) BUILDDIR=$(abspath $(BUILDDIR)) $(build)=Documentation/media clean

Documentation/translations/zh_CN/HOWTO

@@ -149,9 +149,7 @@ Linux内核代码中包含有大量的文档。这些文档对于学习如何与
 核源码的主目录中使用以下不同命令将会分别生成PDF、Postscript、HTML和手册
 页等不同格式的文档：
     make pdfdocs
-    make psdocs
     make htmldocs
-    make mandocs
 
 
 如何成为内核开发者

Makefile

@@ -1467,7 +1467,7 @@ $(help-board-dirs): help-%:
 
 # Documentation targets
 # ---------------------------------------------------------------------------
-DOC_TARGETS := xmldocs sgmldocs psdocs latexdocs pdfdocs htmldocs mandocs installmandocs epubdocs cleandocs linkcheckdocs
+DOC_TARGETS := xmldocs latexdocs pdfdocs htmldocs epubdocs cleandocs linkcheckdocs
 PHONY += $(DOC_TARGETS)
 $(DOC_TARGETS): scripts_basic FORCE
 	$(Q)$(MAKE) $(build)=Documentation $@