Remove unused recovery field in DynamoDB backend

[ungureanuvladvictor]

Looks like when #2141 was merged the recovery field in the DynamoDBLock and DynamoDBBackend structs was not removed but it is not used anymore.

[ungureanuvladvictor]

@jefferai looks like Travis forgot to send back the hook for the failed build. Anyway the build failed because:

ok  	github.com/hashicorp/vault	0.123s [no tests to run]
--- FAIL: TestClientEnvSettings (0.01s)
	client_test.go:167: bad: expected client tls config to have a client certificate
FAIL
FAIL	github.com/hashicorp/vault/api	21.054s

which seems pretty unrelated with my change. https://travis-ci.org/hashicorp/vault/builds/300407400 failed from the same reason as well.

[jefferai]

Thanks!