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

Dockerfile

@@ -10,10 +10,7 @@ RUN go vet ./...
 RUN golangci-lint run --config .golangci.yml ./...
 # The same checks on the code as a macOS build compiles it, which a Linux
 # build never compiles. Cgo is off, because compiling cgo code for macOS
-# needs Apple's SDK headers. That leaves out the files built only with cgo
-# on macOS: the keychain unlocker's calls into the keychain
-# (keychainunlocker_cgo.go, and keychainunlocker_test.go) and the Secure
-# Enclave bindings (internal/macse). Nothing on Linux checks those.
+# needs Apple's SDK headers.
 RUN GOOS=darwin CGO_ENABLED=0 go vet ./...
 RUN GOOS=darwin CGO_ENABLED=0 golangci-lint run --config .golangci.yml ./...