how to properly edit Marlin config to remove unneeded items

i am a complete newb to arduino, obviously. i have a KFB2.0 (Mega chip on a board with everything a 3d printer could use) i flashed it with marlin firmware successfully. but looking at it, theres a bunch of stuff that i dont need , like other boards, anything to do with the 3d printer (extruder, heated bed etc) other LCDs, i just need to have it solely for my setup. im building a plotter so i need X,Y,Z control, my reprap discount LCD , the encoder dependencies ETC. How to i prune the firmware down to just what im running and will only probably run on this board without screwing the whole thing up? i want to be able to trouble shoot without having to parse through other crap i dont even have, to further confuse me and slow my learning

If you need to ask how to do it then you probably should not do it.

Marlin has two configuration files with a lot of #defines. The code for each of those features is controlled by preprocessor directives, for example:
In config.h

//#define SOME_FEATURE

In the Marlin code

#ifdef SOME_FEATURE
// feature code here
#endif

If the SOME_FEATURE macro is not #defined then the preprocessor strips out all that code.

You could spend hours deleting all the code in Marlin you don't use, and hopefully not mess anything up in the process, but keep in mind that Marlin is continuously being improved. You will need to redo all that work any time you want to update to a new version.

It sounds like maybe Marlin is not the best choice. I wonder if something like grbl would be better.