secret: only turn on encryption when the env var is explicitly specified

Created by: unknwon

As of now, the database secrets encryption will be turned on when one of the following happens:

1. The default secret key file exists (/etc/sourcegraph/token).
2. The admin specifies the secret key file via env var SOURCEGRAPH_SECRET_FILE and it exists.

The proposal inspired by my reply to @pecigonzalo is to remove 1. and only keep 2. In addition, when the admin specifies the secret key file via env var SOURCEGRAPH_SECRET_FILE, we throw a hard error when the file does not exist instead of ignoring it.

Would love inputs from @sourcegraph/security about this new proposal before preceding.