chore: remove Sourcehut CI manifest

Migrated to git.swaits.com; Sourcehut builds no longer run.

.build.yml

@@ -1,11 +0,0 @@
-image: alpine/edge
-secrets:
-  - 88cce185-d2ec-4573-9546-e33f36f79bbf
-sources:
-  - git+ssh://git@git.sr.ht/~swaits/hikaru
-tasks:
-  - mirror-to-github: |
-      cd ~/hikaru
-      ssh-keyscan -t rsa github.com >> ~/.ssh/known_hosts
-      git remote add github git@github.com:swaits/hikaru.git
-      git push --mirror github