Config directory cleanup - remove header constants.h it is really never used and pollutes config directory

DO NOT REMOVE THIS BOXED MESSAGE

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File constants.h is in config directory and pretends to be used in file generators/MpdUrqmdGenerator.cxx,
but in reality it doesnt.

It is safe to remove this dependency as it pollutes config dir.
If there will be any physics configuration it should be under config/physics. Main config dir should be for system configuration

There is clang-formatting craziness ongoing in commits (which hides real changes) so here are the changes in short:   
-- generators/MpdUrqmdGenerator.cxx  
   include constants.h line is removed     
   along with the dead code pretending to use those two lines around line 260 is commented out with explanation 
   (in summary gCoordinateSystem is declared as constant sysCenterOfMass and will never be sysLaboratorySystem :D )

-- generators/Mpd3fdGenerator.cxx,  eventdisplay/MpdEventManager.cxx, /generators/MpdSmashGenerator.cxx  
   include constants.h is removed as it is not used  

NOTE: It is really a bad practice (procedural style hack) in general to use such includes. Their content should be either a 
part of a base class, or some general configuration of overall abstract simulation/reconstruction model stored in a singleton. 
(there cannot be for each setting one file like we have constants.h and rootmanager.dat, with different suffix as a bonus -
 such "creativity" is sad and funny at the same time )

ADDITIONAL SMALL CHANGES

/etc/env.sh is moved to /config/env.sh as it is configuration file 
also, by doing this, we get rid of unused /etc directory 

directory /config/vscode.in is removed as it is user specific customization (with additional maintenance burden),
and really should not be in the main codebase.
It can be a part of some tutorial for VS users or some custom project linking to main mpdroot project.

/config/eventdisplay.xml is moved to /eventdisplay directory, which contains other configuration files for old eventdisplay