runtime/tls: remove deprecated package

Remove the runtime/tls package which uses deprecated field names
(certFile, keyFile, caFile) for TLS secrets. This functionality
is superseded by the runtime/secrets package which provides
better API design and supports both standard field names
(tls.crt, tls.key, ca.crt) and deprecated field names via options.

Investigation confirmed that this package is not used by any
FluxCD controllers (source-controller, kustomize-controller,
helm-controller, notification-controller, image-automation-controller)
and can be safely removed.

Signed-off-by: cappyzawa <cappyzawa@gmail.com>

Author: cappyzawa