[doc] Fix minor warning when generating the doc.

Tried a full update but this mix up with current internal doxygeen cmake rules.

Author: olivier-stasse

Diff for: doc/Doxyfile.extra.in

@@ -395,12 +395,6 @@ VERBATIM_HEADERS       = YES
 
 ALPHABETICAL_INDEX     = YES
 
-# If the alphabetical index is enabled (see ALPHABETICAL_INDEX) then
-# the COLS_IN_ALPHA_INDEX tag can be used to specify the number of columns
-# in which this list will be split (can be a number in the range [1..20])
-
-COLS_IN_ALPHA_INDEX    = 4
-
 #---------------------------------------------------------------------------
 # configuration options related to the HTML output
 #---------------------------------------------------------------------------