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/repo.py

@@ -16,7 +16,6 @@ GNU General Public License for more details.
 You should have received a copy of the GNU General Public License
 along with pmbootstrap.  If not, see <http://www.gnu.org/licenses/>.
 """
-import glob
 import os
 import hashlib
 import logging
@@ -24,52 +23,6 @@ import pmb.helpers.http
 import pmb.helpers.run
 
 
-def files(args):
-    """
-    Returns all files (apk/buildinfo) with their last modification timestamp
-    inside the package repository, sorted by architecture.
-
-    :returns: {"x86_64": {"first.apk": last_modified_timestamp, ... }, ... }
-    """
-    ret = {}
-    for arch_folder in glob.glob(args.work + "/packages/*"):
-        arch = os.path.basename(arch_folder)
-        ret[arch] = {}
-        for file in glob.glob(arch_folder + "/*"):
-            basename = os.path.basename(file)
-            ret[arch][basename] = os.path.getmtime(file)
-    return ret
-
-
-def diff(args, files_a, files_b=None):
-    """
-    Returns a list of files, that have been added or modified inside the
-    package repository.
-
-    :param files_a: return value from pmb.helpers.repo.files()
-    :param files_b: defaults to creating a new list
-    :returns: ["x86_64/APKINDEX.tar.gz", "x86_64/package.apk",
-               "x86_64/package.buildinfo", ...]
-    """
-    if not files_b:
-        files_b = files(args)
-
-    ret = []
-    for arch in files_b.keys():
-        for file, timestamp in files_b[arch].items():
-            add = False
-            if arch not in files_a:
-                add = True
-            elif file not in files_a[arch]:
-                add = True
-            elif timestamp != files_a[arch][file]:
-                add = True
-            if add:
-                ret.append(arch + "/" + file)
-
-    return sorted(ret)
-
-
 def hash(url, length=8):
     """
     Generate the hash, that APK adds to the APKINDEX and apk packages