tests: boards: nrf: i2s: Remove test that checks I2S clock divider

Remove test from tests/boards/nrf/i2s/i2s_divider.
The i2s_divider test was relying on debug log from I2S driver.
With the recent NRFX updates log was removed.

Better test, that counts rising edges on I2S_LRCLK line was added
in downstream (https://github.com/nrfconnect/sdk-nrf/pull/26230).

Signed-off-by: Sebastian Głąb <sebastian.glab@nordicsemi.no>