samples: boards: nrfx: Remove redundant inclusions

This is a follow-up to commit c7e4d23c.

After all calls to `nrfx_dppi_*` and `nrfx_ppi_*` functions were
replaced with corresponding `nrfx_gppi_*` calls, it is no longer
needed to include the `nrfx_dppi.h` and `nrfx_ppi.h` headers.

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