Fix: destroy session removes the default config file

Destroy session command by default removes the default config file
without checking the current session. As a result when we call any
other command which expects a default session by calling
get_session_name() function, it fails.

This patch will fix this by checking that the default config file gets
removed only when destroy session is called with the current session.

Fixes: #887
Signed-off-by: Partha Pratim Mukherjee <ppm.floss@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>