This hasn't worked in a while, presumably since one of our upstream
merges.  Don't bother making it work, since MD2 is generally considered
insecure.

There was some... frankly kind of scary buffer and data shuffling if
ESP8266_PLATFORM was defined.  Since we don't, in fact, define that
preprocessor symbol, just drop the code lest anyone (possibly future-me)
be scared.

No functional changes

There's no need to malloc a structure that's used only locally.

What an absolute shitshow this is.  mbedtls should absolutely not
be mentioned inside sys/socket.h and app/mbedtls/app/lwIPSocket.c is not
so much glue as it as a complete copy of a random subset of lwIP; it
should go, but we aren't there yet.

Get rid of the mysterious "mbedlts_record" struct, which housed merely a
length of bytes sent solely for gating the "record sent" callback.

Remove spurious __attribute__((weak)) from symbols not otherwise
defined and rename them to emphasize that they are not actually part of
mbedtls proper.

This at least makes the shitshow smaller

I presume these also need the new arguments

Depending on who you ask it's either EOL already or EOL soon, so
we may as well get rid of it now.