Drop the legacy build-host names

OMARCHY_BUILD_HOST and .build-host were carried as fallbacks through the
rename. Nothing in this checkout ever set either one, so they were a second
name for a setting that has only one real name.

Resolution is now --host, OMARCHY_REPO_HOST, .repo-host.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -33,5 +33,4 @@ pkgbuilds/symfony-cli/symfony*
 !pkgbuilds/symfony-cli/symfony-cli.install
 pkgbuilds/yay/yay/
 .srcdest/
-.build-host
 .repo-host