Remove deprecated tags from Doxyfile.in

This fixes warnings emitted by Doxygen 1.8.16.

docs/Doxyfile.in

@@ -1632,11 +1632,6 @@ ALLEXTERNALS           = NO
 
 EXTERNAL_GROUPS        = YES
 
-# The PERL_PATH should be the absolute path and name of the perl script
-# interpreter (i.e. the result of `which perl').
-
-PERL_PATH              = /usr/bin/perl
-
 #---------------------------------------------------------------------------
 # Configuration options related to the dot tool
 #---------------------------------------------------------------------------
@@ -1649,15 +1644,6 @@ PERL_PATH              = /usr/bin/perl
 
 CLASS_DIAGRAMS         = YES
 
-# You can define message sequence charts within doxygen comments using the \msc
-# command. Doxygen will then run the mscgen tool (see
-# http://www.mcternan.me.uk/mscgen/) to produce the chart and insert it in the
-# documentation. The MSCGEN_PATH tag allows you to specify the directory where
-# the mscgen tool resides. If left empty the tool is assumed to be found in the
-# default search path.
-
-MSCGEN_PATH            =
-
 # If set to YES, the inheritance and collaboration graphs will hide
 # inheritance and usage relations if the target is undocumented
 # or is not a class.