treewide: revert sysupgrade adjustments for early DSA-adopters

treewide: revert sysupgrade adjustments for early DSA-adopters

The uci-default mechanism to update the compat-version was only
meant for early DSA-adopters, which should have updated by now.

Remove this workaround again in order to prevent the intended
experiences for all the other people.

This reverts:
a9703db72030 ("mvebu: fix sysupgrade experience for early DSA-adopters")
86c89bf5e8f5 ("kirkwood: fix sysupgrade experience for early DSA-adopters")

Partially reverted:
1eac573b5304 ("ramips: mt7621: implement compatibility version for DSA migration")

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>