Remove 'tinycrypt' from MCUboot

[d3zd3z]

This code has no upstream maintenance (basically abandoned), and the implementation in Mbed TLS is now better, anyway. I believe it is time we remove this code entirely from MCUboot.

If there are concerns about code-size and PSA, one possibility is to use the P256m code from Mbed TLS directly. It is quite a bit smaller as well as faster than tinycrypt. The other functionality used from tinycrypt, namely hashing, should be readily available.