Remove vestiges of attempt at MS Crypto API support.

There was a time, back when the USA was more vigorously against
cryptography, when we toyed with the idea of having a version of PuTTY
that outsourced its cryptographic primitives to the Microsoft optional
encryption API, which would effectively create a tool that acted like
PuTTY proper on a system with that API installed, but automatically
degraded to being PuTTYtel on a system without, and meanwhile (so went
the theory) it could be moved freely across national borders with
crypto restrictions, because it didn't _contain_ any of the actual
crypto.

I don't recall that we ever got it working at all. And certainly the
vestiges of it here and there in the current code are completely
unworkable - they refer to an 'mscrypto.c' that doesn't even exist,
and the ifdefs in the definitions of structures like RSAKey and
MD5Context are not matched by any corresponding ifdefs in the code. So
I ought to have got round to removing it long ago, in order to avoid
misleading anyone.

putty.h

@@ -1259,14 +1259,6 @@ int mk_wcswidth(const unsigned int *pwcs, size_t n);
 int mk_wcwidth_cjk(unsigned int ucs);
 int mk_wcswidth_cjk(const unsigned int *pwcs, size_t n);
 
-/*
- * Exports from mscrypto.c
- */
-#ifdef MSCRYPTOAPI
-int crypto_startup();
-void crypto_wrapup();
-#endif
-
 /*
  * Exports from pageantc.c.
  *