Remove rest of 'pmbootstrap challenge' left overs (#1173)

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).
* `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.

pmb/helpers/frontend.py

@@ -144,7 +144,7 @@ def build(args):
     for package in args.packages:
         arch_package = args.arch or pmb.build.autodetect.arch(args, package)
         if not pmb.build.package(args, package, arch_package, args.force,
-                                 args.buildinfo, args.strict):
+                                 args.strict):
             logging.info("NOTE: Package '" + package + "' is up to date. Use"
                          " 'pmbootstrap build " + package + " --force'"
                          " if needed.")