drivers: i2c_nrfx_twim: Remove extra path for zero-length transfers

nrfx 2.5.0 release includes the patch in the nrfx_twim driver that
fixes the driver behavior for zero-length transfers. No need to keep
the same fix in the shim layer.
This effectively reverts cb86a2b3.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>