Remove rest of 'pmbootstrap challenge' left overs

Created by: ollieparanoid

Follow up to #1162.

• pmb.build.buildinfo(): Used to record the build environment. It is flawed because it scans the repo APKINDEX files instead of using the actually installed packages list. When it was implemented we were not able to do the latter. After this is removed, pmb.parse.depends can be simplified (it needs to be rewritten for #1122 (closed)).
• pmb.helpers.repo.diff() and pmb.helpers.repo.files(): These were used exclusively by pmb.build.buildinfo(), to learn about which files have been changed in the local repository folder after a package was built. The idea was, that we could find subpackages that way. But this information is present in the installed package list as well, which is a much cleaner approach.