Fix #3144: check for promptDeleteAllFiles config setting before emitting signal to...

Fix #3144: check for promptDeleteAllFiles config setting before emitting signal to display warning.

The signal is not connected to any slot when running nextcloudcmd.
The callback to finish the sync was never being called because the check for the config was done in the slot,
which was never called.

Signed-off-by: Camila Ayres <hello@camilasan.com>