mbedtls: remove unnecessary headers

These headers are basically a copy of those found in the official
Mbed TLS repo in the "configs" folder. However these are unmaintaned
(i.e. not updated when Mbed TLS is bumped to a new version) and
also unused in Zephyr. Therefore there's no benefit in keeping
them around.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>