Remove Doxygen options obsolete in 1.9.1

We don't use any of these options anyhow, so just remove them to avoid
warnings about them being obsolete now.

docs/doxygen/Doxyfile

@@ -29,7 +29,6 @@ MULTILINE_CPP_IS_BRIEF = NO
 INHERIT_DOCS           = YES
 SEPARATE_MEMBER_PAGES  = NO
 TAB_SIZE               = 4
-TCL_SUBST              =
 OPTIMIZE_OUTPUT_FOR_C  = NO
 OPTIMIZE_OUTPUT_JAVA   = NO
 OPTIMIZE_FOR_FORTRAN   = NO
@@ -344,7 +343,6 @@ VERBATIM_HEADERS       = NO # Default: YES
 #---------------------------------------------------------------------------
 
 ALPHABETICAL_INDEX     = YES
-COLS_IN_ALPHA_INDEX    = 5
 IGNORE_PREFIX          = wx
 
 
@@ -545,7 +543,6 @@ GENERATE_TAGFILE       = $(GENERATE_TAGFILE)
 ALLEXTERNALS           = NO
 EXTERNAL_GROUPS        = YES
 EXTERNAL_PAGES         = YES
-PERL_PATH              = /usr/bin/perl
 
 
 #---------------------------------------------------------------------------
@@ -553,7 +550,6 @@ PERL_PATH              = /usr/bin/perl
 #---------------------------------------------------------------------------
 
 CLASS_DIAGRAMS         = YES
-MSCGEN_PATH            =
 DIA_PATH               =
 HIDE_UNDOC_RELATIONS   = YES
 HAVE_DOT               = YES # Default: NO