MINOR: session: remove redundant target argument from session_add_conn()

MINOR: session: remove redundant target argument from session_add_conn()

session_add_conn() uses three argument : connection and session
instances, plus a void pointer labelled as target. Typically, it
represents the server, but can also be a backend instance (for example
on dispatch).

In fact, this argument is redundant as <target> is already a member of
the connection. This commit simplifies session_add_conn() by removing
it. A BUG_ON() on target is extended to ensure it is never NULL.