Remove old legacy code

[mkg20001]

Buildozer seems to have improved greatly, thus removing the need for some hacks/workarounds as well as the forks.

list WIP

• [x] Workaround for downloading newer SDKs/NDKs: ?
• [x] Workaround for buildozer sometimes not properly fetching stuff: Removed
• [x] $CI_MODE feature: Possibly get this merged upstream (https://github.com/kivy/python-for-android/pull/1727 and https://github.com/kivy/python-for-android/pull/1727)
• [x] libjpeg fix: Merged upstream
• [x] Consider using offical docker image, or syncing with it
• [x] Wrapper command for docker buildozer: Update
• [x] Makefile cleanup