lib/libc/Kconfig: Fix libC depends with NATIVE_APPlICATION

NATIVE_APPlICATION (old native_posix) does not support bulding with the
any other library than the host libC (EXTERNAL_LIBC).

MINIMAL_LIBC_SUPPORTED defaulted to y always, and instead the depends
was set in the MINIMAL_LIBC choice.
This lead to the right library selected, but an incorrectly set
MINIMAL_LIBC_SUPPORTED.
Many tests filter based on MINIMAL_LIBC_SUPPORTED, so they were
not filtered out appropriately.

Let's place the depends in the right place to avoid this problem.
Also remove a redundant depends on for PICOLIBC and NEWLIB_LIBC
which already have the dependency in their respective _SUPPORTED
option.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>