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/secret/metadata.go

@@ -17,7 +17,7 @@ type VaultMetadata struct {
 
 // UnlockerMetadata contains information about an unlocker
 type UnlockerMetadata struct {
-	Type      string    `json:"type"` // passphrase, pgp, keychain
+	Type      string    `json:"type"` // passphrase or pgp
 	CreatedAt time.Time `json:"createdAt"`
 	Flags     []string  `json:"flags,omitempty"`
 }