Remove the keychain and Secure Enclave unlockers (closes #132)

Neither unlocker works without macOS code signing and entitlements, which
secret does not have, so both were research and leave the build.
`secret unlocker add` takes only passphrase and pgp on every platform;
keychain and secure-enclave fail as unknown types and are gone from
--help and shell completion. internal/macse, their files in
internal/secret with their tests, and their branches in internal/vault
and internal/cli are deleted; github.com/keybase/go-keychain leaves
go.mod and go.sum. README and Dockerfile comments no longer mention
them. The research stays at ef0ae90, tagged v1.0.0-rc1.

Judgement call: derivation_index_test.go goes with getLongTermPrivateKey,
its only subject, which only these two unlockers used.
Unverified: not built or run on a Mac; only the GOOS=darwin vet and lint ran.

Model: opus-5-5

internal/vault/unlockers.go

@@ -18,8 +18,7 @@ import (
 
 // Unlocker metadata type strings.
 const (
-	unlockerTypePassphrase    = "passphrase"
-	unlockerTypeSecureEnclave = "secure-enclave"
+	unlockerTypePassphrase = "passphrase"
 )
 
 // GetCurrentUnlocker returns the current unlocker for this vault
@@ -79,15 +78,6 @@ func (v *Vault) GetCurrentUnlocker() (secret.Unlocker, error) {
 		secret.Debug("Creating PGP unlocker instance", "unlocker_type", metadata.Type)
 
 		unlocker = secret.NewPGPUnlocker(v.fs, unlockerDir, metadata)
-	case "keychain":
-		secret.Debug("Creating keychain unlocker instance", "unlocker_type", metadata.Type)
-
-		unlocker = secret.NewKeychainUnlocker(v.fs, unlockerDir, metadata)
-	case unlockerTypeSecureEnclave:
-		secret.Debug("Creating secure enclave unlocker instance",
-			"unlocker_type", metadata.Type)
-
-		unlocker = secret.NewSecureEnclaveUnlocker(v.fs, unlockerDir, metadata)
 	default:
 		secret.Debug("Unsupported unlocker type", "type", metadata.Type)
 
@@ -171,10 +161,6 @@ func (v *Vault) findUnlockerByID(
 			tempUnlocker = secret.NewPassphraseUnlocker(v.fs, unlockerDirPath, metadata)
 		case "pgp":
 			tempUnlocker = secret.NewPGPUnlocker(v.fs, unlockerDirPath, metadata)
-		case "keychain":
-			tempUnlocker = secret.NewKeychainUnlocker(v.fs, unlockerDirPath, metadata)
-		case unlockerTypeSecureEnclave:
-			tempUnlocker = secret.NewSecureEnclaveUnlocker(v.fs, unlockerDirPath, metadata)
 		default:
 			continue
 		}