cifs: remove redundant variable tcon_exist

The variable tcon_exist is being assigned however it is never read, the
variable is redundant and can be removed.

Cleans up clang scan build warning:
warning: Although the value stored to 'tcon_exist' is used in
the enclosing expression, the value is never actually readfrom
'tcon_exist' [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Steve French <stfrench@microsoft.com>