Revert "ci: switch to pmbootstrap v3 (MR 5566)"

As of
https://gitlab.com/postmarketOS/ci-common/-/commit/666df6d1333ee1699d13bf6f75ab0f66b78aaa42
the default pmbootstrap version used is master, not 2.3.x. As such,
let's avoid duplicating that downstream.

This reverts commit 7a930582.

.gitlab-ci.yml

@@ -10,8 +10,6 @@ stages:
   - lint
   - build
   - autoupdate
-variables:
-  PMBOOTSTRAP_TAG: "master"
 
 # This defines the rules for when a pipeline should run.
 workflow: