samples: net: irc_bot: simplify connect path

K_FOREVER timeout in net_context_connect() will wait until
SYNACK has been received and we're ready to proceed with socket
communication.

To make the code as clear as possible, let's remove 2 callback
functions and relocate that code inside zirc_connect().

Change-Id: I6c64852ad5f48b24e1e6a58a741981e7d07bfd40
Signed-off-by: Michael Scott <michael.scott@linaro.org>