systemd: drop usr-merge

[caleb: adding context] This enables using systemd with split /usr, it is intended to be a temporary solution to allow our systemd work to continue while the /usr merge is implemented upstream in Alpine. We previously attempted to work around this by patching apk-tools, but it's become clear that this is not a viable solution, and given the rate at which /usr merges patches are getting merged in Alpine it isn't worth investing in it further. The /usr merge is still a hard requirement for us and will be a blocker to ship systemd in postmarketOS stable.

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one way to test this:

git checkout origin/jane400/systemd/split-usr-bin
pmbootstrap config mirrors.systemd "none"
pmbootstrap repo_bootstrap systemd
# can't activate the mirrors.systemd back on, apk-tools would get pulled in
pmbootstrap install