boards: esp32-based: unset default libc in appcpu

As in all other esp32-based boards, APPCPU
does not require to have MINIMAL_LIBC as default libc.
In the past, that was required due to memory constraints,
which is not the case anymore.

This fixes build issues in `tests/lib/c_lib/common`.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>