Remove "pmbootstrap repo_bootstrap"

The repo bootstrap logic has worked well enough in the initial systemd
bringup. But we have decided to replace the approach of building
packages multiple times during the repo_bootstrap with building each
package only once and adding -stage0 packages where necessary (currently
only a systemd-stage0 package).

Advantages:
* Replace the often breaking repo_bootstrap logic with something less
  fragile (repo_bootstrap is currently broken again).
* Can get rid of the whole bootstrap logic in pmaports.cfg, pmbootstrap,
  build.postmarketos.org. This will make pmbootstrap and bpo easier to
  maintain.
* Fix problems we have seen when upgrading two or more of (systemd,
  dbus, linux-pam-pmos) at once: it doesn't pass in CI and it doesn't
  pass in BPO
* You don't need to do a whole bootstrap at once, it is again broken
  down by package. This means if building one package fails (locally or
  in bpo), we don't need to do the whole thing again, just start at the
  package that failed. This also means it is much easier to optimize
  e.g. the stage0 packages to build faster.
* Fixes some specific bugs we currently have (abuild twice in pmaports,
  bpo tests disabled because of that, ...)

Part-of: https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/merge_requests/2588

docs/api/pmb.commands.rst

@@ -76,14 +76,6 @@ pmb.commands.pull module
    :undoc-members:
    :show-inheritance:
 
-pmb.commands.repo_bootstrap module
-----------------------------------
-
-.. automodule:: pmb.commands.repo_bootstrap
-   :members:
-   :undoc-members:
-   :show-inheritance:
-
 pmb.commands.shutdown module
 ----------------------------