fix(nix): the rename lives with the module that declares the options

mara: "remove the extra deprecatios file, less comments". The alias now
sits in hive-forge/default.nix next to the options it renames, and the
comment is three lines instead of eighteen.

Better than tidier: there is no shared file for the sibling migrations
to queue behind. Matrix's rename goes in hive-matrix.nix, authelia's in
its own module, and none of them touch each other. hive-ci.nix declares
swarm.forge.ci.* from a different module and stays covered, because the
rename is on the namespace rather than on the declaring file.

Re-ran the check both ways this time, per argus on the last round: old
paths set and new paths read back 3999 / 7, and a config that only uses
the new paths produces no warnings at all.

nix/host-modules/default.nix

@@ -12,7 +12,6 @@
 {
   imports = [
     ./hyperhive.nix
-    ./swarm-renames.nix
     ./hive-c0re
     ./hive-ci.nix
     ./hive-forge