doc: remove Doxygen configuration

Signed-off-by: VirtualTam <virtualtam@flibidi.net>

.gitignore

@@ -22,7 +22,6 @@ inc/languages/*/LC_MESSAGES/shaarli.mo
 
 # Development and test resources
 coverage
-doxygen
 sandbox
 phpmd.html
 phpdoc.xml

Makefile

@@ -146,11 +146,6 @@ authors:
 	@git shortlog -sne > AUTHORS
 	@rm .mailmap
 
-### generate Doxygen documentation
-doxygen: clean
-	@rm -rf doxygen
-	@doxygen Doxyfile
-
 ### generate phpDocumentor documentation
 phpdoc: clean
 	@docker run --rm -v $(PWD):/data -u `id -u`:`id -g` phpdoc/phpdoc