Additionally remove the KMSv1 Reserved EC Key in KmsKeyring

[kessplas]

In Java:

    private static final String KEY_ID_CONTEXT_KEY = "kms_cmk_id";

            //= specification/s3-encryption/materials/s3-kms-keyring.md#kms-context
            //# The stored encryption context with the two reserved keys removed MUST match the provided encryption context.
            materialsEncryptionContextCopy.remove(KEY_ID_CONTEXT_KEY);
            materialsEncryptionContextCopy.remove(ENCRYPTION_CONTEXT_ALGORITHM_KEY);

#143 points out we don't currently do this in Python.