Remove the dead admin-revoker tool (#7941)

The admin-revoker tool is dead. Long live the admin tool.

There's a number places that still reference admin-revoker, including
Boulder's ipki and the revocation source in the database which are still
used, even if the tool is gone. But nothing actually using the tool.

cmd/boulder/main.go

@@ -5,7 +5,6 @@ import (
 	"os"
 	"strings"
 
-	_ "github.com/letsencrypt/boulder/cmd/admin-revoker"
 	_ "github.com/letsencrypt/boulder/cmd/akamai-purger"
 	_ "github.com/letsencrypt/boulder/cmd/bad-key-revoker"
 	_ "github.com/letsencrypt/boulder/cmd/boulder-ca"

ra/ra.go

@@ -1969,7 +1969,7 @@ func (ra *RegistrationAuthorityImpl) RevokeCertByKey(ctx context.Context, req *r
 
 // AdministrativelyRevokeCertificate terminates trust in the certificate
 // provided and does not require the registration ID of the requester since this
-// method is only called from the admin-revoker tool. It trusts that the admin
+// method is only called from the `admin` tool. It trusts that the admin
 // is doing the right thing, so if the requested reason is keyCompromise, it
 // blocks the key from future issuance even though compromise has not been
 // demonstrated here. It purges the certificate from the Akamai cache, and