network: properly remove connections from list

Due to a typo we were always removing the first (index 0) connection
from the global list of connections instead of the correct one.

This resulted in some connections remaining in the shell's connection
list long after they were removed.  In particular, this resulted in
multiple copies of a bluetooth connection appearing after suspend/resume
(when the device was readded and the cached connection list was
rescanned).

https://bugzilla.gnome.org/show_bug.cgi?id=740227
(cherry picked from commit 3d4408dce810f3b6995ed2acf28b61bed48c1dac)