[gtkmm] docs/reference/Doxyfile.in: Remove obsolete entry

[Kjell Ahlstedt <kjellahl src gnome org>]

commit aec62dfa813f39ffbe762a4f29d1047d57d5baac
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Wed Sep 8 15:30:44 2021 +0200

    docs/reference/Doxyfile.in: Remove obsolete entry

 docs/reference/Doxyfile.in | 1 -
 1 file changed, 1 deletion(-)
---
diff --git a/docs/reference/Doxyfile.in b/docs/reference/Doxyfile.in
index 5ceabeab..1eb35495 100644
--- a/docs/reference/Doxyfile.in
+++ b/docs/reference/Doxyfile.in
@@ -156,7 +156,6 @@ CLANG_OPTIONS          =
 # Configuration options related to the alphabetical class index
 #---------------------------------------------------------------------------
 ALPHABETICAL_INDEX     = YES
-COLS_IN_ALPHA_INDEX    = 5
 IGNORE_PREFIX          =
 #---------------------------------------------------------------------------
 # Configuration options related to the HTML output