doc - drop stale DOXYGEN option

Author: jeremylt

Doxyfile

@@ -1127,7 +1127,7 @@ FILTER_SOURCE_PATTERNS =
 # (index.html). This can be useful if you have a project on for instance GitHub
 # and want to reuse the introduction page also for the doxygen output.
 
-USE_MDFILE_AS_MAINPAGE = ./README.md
+USE_MDFILE_AS_MAINPAGE =
 
 # The Fortran standard specifies that for fixed formatted Fortran code all
 # characters from position 72 are to be considered as comment. A common