tipc: Remove unused global variable tipc_user_count

Eliminates a global variable that was previously used by TIPC's user
registry to track the number of distinct applications using TIPC. Due to
the recent elimination of the user registry this variable no longer serves
any purpose and can be removed.

Signed-off-by: Allan Stephens <Allan.Stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>