Fix a few warnings related to unused variables

[jmcarcell]

BEGINRELEASENOTES

• Fix a few warnings related to unused variables
• Remove a comment that doesn't make sense anymore without the removed variables

ENDRELEASENOTES

Warnings like:

/Marlin/source/src/SimpleFastMCProcessor.cc:198:48: warning: unused parameter 'evt' [-Wunused-parameter]
  198 |   void SimpleFastMCProcessor::check( LCEvent * evt ) {
      |                                                ^
1 warning generated.
[94/128] Building CXX object Marlin/source/CMakeFiles/Marlin.dir/src/XMLParser.cc.o
/Marlin/source/src/XMLParser.cc:198:18: warning: variable 'procCount' set but not used [-Wunused-but-set-variable]
  198 |         unsigned procCount(0) ;
      |                  ^
/Marlin/source/src/XMLParser.cc:199:18: warning: variable 'typedProcCount' set but not used [-Wunused-but-set-variable]
  199 |         unsigned typedProcCount(0) ;
      |                  ^

[tmadlener]

The changes that would fix the warning in the SimpleFastMCProcessor are now gone it seems?

[jmcarcell]

Brought back, I'm not sure it's always necessary though since it depends if MARLIN_AIDA is defined or not.

[tmadlener]

Unless there are more, I would merge.

[jmcarcell]

I don't see any more, apparently the ones in #53 do not happen anymore. I probably have a Clang 2 or 3 versions newer.