Delete deprecated doxygen options

These two options are deprecated in doxygen (don't know from which version)

This deletes two warnings :
```
Warning: Tag `SYMBOL_CACHE_SIZE' at line 81 of file /home/dell/pcl/build/doc/doxygen/doxyfile has become obsolete.
To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
Warning: Tag `SHOW_DIRECTORIES' at line 118 of file /home/dell/pcl/build/doc/doxygen/doxyfile has become obsolete.
To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
```

There is still an error I don't understand (it was there before the change) :
```
error: the type 'dirs' is not supported for the entry tag within a navindex! Check your layout file!
```