Remove clean-local not to remove generated man pages with `make clean`

Because we don't require sphinx-doc to build distributed archive, the
man pages are generated when making distribution. And We want to keep
the generated man pages with `make clean` there.

doc/manual-src/en/Makefile.am

@@ -141,5 +141,3 @@ doctest:
 	$(SPHINXBUILD) -b doctest $(ALLSPHINXOPTS) $(BUILDDIR)/doctest
 	@echo "Testing of doctests in the sources finished, look at the " \
 	      "results in $(BUILDDIR)/doctest/output.txt."
-
-clean-local: docclean

doc/manual-src/pt/Makefile.am

@@ -137,5 +137,3 @@ doctest:
 	$(SPHINXBUILD) -b doctest $(ALLSPHINXOPTS) $(BUILDDIR)/doctest
 	@echo "Testing of doctests in the sources finished, look at the " \
 	      "results in $(BUILDDIR)/doctest/output.txt."
-
-clean-local: docclean

doc/manual-src/ru/Makefile.am

@@ -137,5 +137,3 @@ doctest:
 	$(SPHINXBUILD) -b doctest $(ALLSPHINXOPTS) $(BUILDDIR)/doctest
 	@echo "Testing of doctests in the sources finished, look at the " \
 	      "results in $(BUILDDIR)/doctest/output.txt."
-
-clean-local: docclean