Fix warnings: spell.c: find_word(): Dead assignment: HI.

Problem    : Dead assignment @ 1602.
Diagnostic : Harmless issue.
Rationale  : Code using this assignment (line 1666) was disabled. Vim's
             tip at Wed Nov 12 13:07:54 2014 +0100 (changeset
             6352:2f7bf5f90f57) hasn't changed this yet.
Resolution : Disable assignment. Directive processors are used for that
             in order to match the way the other code was disabled.

src/nvim/spell.c

@@ -1599,7 +1599,9 @@ static void find_word(matchinf_T *mip, int mode)
             mip->mi_compoff = (int)(p - mip->mi_fword);
           }
         }
+#if 0
         c = mip->mi_compoff;
+#endif
         ++mip->mi_complen;
         if (flags & WF_COMPROOT)
           ++mip->mi_compextra;