samples: mgmt: updatehub: Remove GSM modem overlay

The gsm modem driver is deprecated and will be removed. Remove
the build overlay and dependency on the gsm modem from the
updatehub sample.

The new cellular modem driver and subsystem use the native
networking stack and the pm subsystem, so just like ethernet,
enabling the cellular modem is up to the board configuration.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>