UBUNTU: SAUCE: xr-usb-serial: remove driver

UBUNTU: SAUCE: xr-usb-serial: remove driver

BugLink: https://bugs.launchpad.net/bugs/1945938
This custom driver was added in Xenial LTS kernel to add support for USB
UART chips:
Product Family: USB UART
    Part Numbers:
    	XR21V1410, XR21V1412, XR21V1414,
    	XR21B1411, XR21B1420, XR21B1422,
    	XR21B1424, XR22801, XR22802,
    	XR22804

In 5.13.x, support for these devices was added to the kernel via.
following patch series:
Link: https://lore.kernel.org/all/20210412095557.1213-1-johan@kernel.org/
Therefore, let's remove this redundant out-of-tree driver.

Signed-off-by: Shrirang Bagul <shrirang.bagul@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>