tty: audit: remove unused variable

While building with W=1 we were getting build warning:
drivers/tty/tty_audit.c:149:16: warning: variable 'sessionid' set but not used

The local variable sessionid was only assigned the value of
current->sessionid but was never reused. On further inspection it turned
out that there is no need of audit_get_loginuid() also.

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>