mbedtls: remove unused kconfigs for Blowfish, ARC4 and MD4

As of Mbed TLS 3.6 (which is the currently used version in Zephyr)
Blowfish, ARC4 and MD4 are no more supported so existing kconfigs
have no effect at all. Moreover they are not even used anywhere
in Zephyr.
This commit just removes them all and it also updates the migration
guide document.

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