doc/doxygen: remove dots/ directory from doxyfile

Author: AnnsAnns

doc/doxygen/riot.doxyfile

@@ -2618,7 +2618,7 @@ DOT_PATH               =
 # command).
 # This tag requires that the tag HAVE_DOT is set to YES.
 
-DOTFILE_DIRS           = dot/
+DOTFILE_DIRS           =
 
 # The MSCFILE_DIRS tag can be used to specify one or more directories that
 # contain msc files that are included in the documentation (see the \mscfile