[core] Remove erroneous "assert !connected" in connect()
Connections like XMPPTCPConnection may still reported connected, if
they are, for example disconnected but resumable. This is already
accounted for in throwAlreadyConnectedExceptionIfAppropriate(), hence
the assert is unnecessary and leads to false negatives for
XMPPTCPConnection.
For the sake of completeness, the right condition, assuming
XMPPTCPConnection is used, for the assert would be:
