Various Makefile optimisations

[zvecr]

Description

Some perf. some tidy up.

• Remove more unused content
• Avoid as much as possible when running simple commands like make clean
  • Be lazier when handling test discovery
  • No point generating version.h
• Align location of phony targets

Terrible perf testing...

msys before

$ time make clean
QMK Firmware 0.15.11
Deleting .build/ ... done.

real    0m5.124s
user    0m0.122s
sys     0m0.288s

msys after

$ time make clean
QMK Firmware 0.15.11
Deleting .build/ ... done.

real    0m0.322s
user    0m0.075s
sys     0m0.166s

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).