samples: i2s: echo: Remove DT_COMPAT_<compat> defines

We auto-generate DT_COMPAT_<compat> defines so we dont need to
explicitly define them anymore.

NOTE: we need to source Kconfig.zephyr first to pull in the autogen
Kconfig.dts files.

Signed-off-by: Kumar Gala <galak@kernel.org>