Remove SecureCredentialsProvider, use JGit's UsernamePasswordCredentialsProvider

The SecureCredentialsProvider class actually had nothing to do with the
user/pass from the secure.config as its name suggests. It was just
another implementation of a user/pass credentials provider. Since JGit
already provides a standard implementation,
UsernamePasswordCredentialsProvider, prefer to use that one.

Change-Id: Iedd196f8bcec15328b4b0c15afe399e3e19bfa70