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/cli/unlockers_ids_test.go

@@ -19,8 +19,7 @@ import (
 // copying an unlocker directory leaves them. It asserts that `unlocker
 // list` and the shell completion of `unlocker select` and `unlocker remove`
 // give each its own ID, and that each is selected and removed by its ID
-// alone. Keychain and Secure Enclave unlockers, which only macOS can add,
-// get their IDs the same way.
+// alone.
 func TestSameMetadataUnlockersHaveTheirOwnIDs(t *testing.T) {
 	t.Parallel()