some clean up

1. delete the COMPILING file as it is heavily outdated

• macos instructions were not edited in 4 years
• linux and windows instructions were not edited in 9 years

and almost everything in this file is now obsolete or wrong

build instructions in README.md file are meant to be up to date

keeping this obsolete file is misleading, ref #133 (closed)

2. fix the netradiant target in README.md file, fix #134 (closed)

also mention the BUNDLE_LIBRARIES cmake option, ref #133 (closed)

3. rename game_packs target to gamepacks, ref #134 (closed)