i2c: i801: Drop duplicate NULL check in i801_del_mux()

Since gpiod_remove_lookup_table() is NULL-aware, no need to have this check
in the caller. Drop duplicate NULL check.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>