Remove lots of legacy 'challenge' code (#1162)

Preparation for #1122.

* `pmb.parse.apkindex.parse()`, removed strict parameter: This used to raise
  an exception when two entries in the apkindex provided the same package.
  Turns out this is *not* invalid after all, two packages can provide the same
  soname for example (e.g. libhybris, mesa-egl). In an APKINDEX, sonames are
  listed as they were packages ("so:libjpeg.so.8" etc.).
* Remove `pmbootstrap challenge` leftover code from reproducible builds effort,
  which was a dead end. This code uses the broken strict feature.