drivers: ethernet: remove dedicated drivers__ethernet__native_posix lib

Fixes: #38403

The two eth_native_posix.c and eth_native_posix_adapt.c are now added
to the common drivers__ethernet Zephyr library.

Instead of creating a dedicated library for just two files those files
are now added to the common ethernet library, see also #8826.
Instead, the dedicated compile definitions required for those files are
specified using COMPILE_DEFINITIONS on the source files.

This also avoids the following warning as the ethernet library is no
longer empty.

> No SOURCES given to Zephyr library: drivers__ethernet
>
> Excluding target from build.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>