Remove a few completely unused functions.

MD5_Simple and SHA512_Simple are superseded by the general
hash_simple() in sshauxcrypt.c. The SHA-1 and SHA-256 analogues of
those functions were already removed as a side effect of reorganising
their source files for hardware acceleration, but the hashes I didn't
rewrite managed to keep their 'Simple' APIs.

mp_neg was never used in the first place. I wrote it as an obvious
companion to mp_neg_into, but since I never put either of those in the
header file, none of the rest of the code ever called them; and
internally to mpint.c, only mp_neg_into is used out of the two.

(Which makes sense, because mp_int is fundamentally an unsigned type,
so mp_neg has weird semantics - it will give you not -x but 2^k-x for
some not-entirely-specified k. mp_neg_into is actually more coherent,
because _you_ get to control k by your choice of the target mp_int.)

mpint.c

@@ -899,13 +899,6 @@ mp_int *mp_sub(mp_int *x, mp_int *y)
     return r;
 }
 
-mp_int *mp_neg(mp_int *a)
-{
-    mp_int *r = mp_make_sized(a->nw);
-    mp_neg_into(r, a);
-    return r;
-}
-
 /*
  * Internal routine: multiply and accumulate in the trivial O(N^2)
  * way. Sets r <- r + a*b.