shell: exchange k_poll for k_event

The shell subsystem currently uses k_poll for signalling. However,
k_poll is only used for simple event signals, results are not used.

Replacing the k_poll with k_event greatly simplifies the code, and
saves 4 struct k_poll_signal and 4 struct k_poll_event (one of which
was entirely unused) while costing a single struct k_event, for
every shell instance. It also allows us to not select POLL,
as we are using the simpler EVENTS instead.

A quick test build of the shell test suite on an nrf54l15 produces
the following build info:

using EVENTS:

           FLASH:       71592 B      1428 KB      4.90%
             RAM:        9872 B       188 KB      5.13%
        IDT_LIST:          0 GB        32 KB      0.00%

using POLL

           FLASH:       75524 B      1428 KB      5.16%
             RAM:       11224 B       188 KB      5.83%
        IDT_LIST:          0 GB        32 KB      0.00%

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>