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/constants.go

@@ -18,9 +18,9 @@ const (
 	EnvGPGKeyID = "SB_GPG_KEY_ID"
 
 	// UnlockerTimeFormat is the layout of the time, in UTC, in the name of a
-	// new unlocker's directory, keychain item and Secure Enclave key. It runs
-	// to the nanosecond, so that every new unlocker, even one added right
-	// after another, gets a directory of its own.
+	// new unlocker's directory. It runs to the nanosecond, so that every new
+	// unlocker, even one added right after another, gets a directory of its
+	// own.
 	UnlockerTimeFormat = "2006-01-02.15.04.05.000000000"
 )