Remove option SHOW_DIRECTORIES

It has been obsolete since Doxygen 1.8.1

doc/doxygen/Doxystyle

@@ -107,8 +107,6 @@ INTERNAL_DOCS          = YES
 # then setting the SHOW_DIRECTORIES tag to YES will show the directory hierarchy
 # in the documentation. The default is NO.
 
-SHOW_DIRECTORIES       = YES
-
 REFERENCED_BY_RELATION = YES
 REFERENCES_RELATION    = YES
 ALPHABETICAL_INDEX     = YES
@@ -207,7 +205,6 @@ GENERATE_BUGLIST       = YES
 GENERATE_DEPRECATEDLIST= YES
 ENABLED_SECTIONS       = 
 MAX_INITIALIZER_LINES  = 30
-SHOW_DIRECTORIES       = YES
 SHOW_FILES             = YES
 SHOW_NAMESPACES        = YES
 FILE_VERSION_FILTER    =