sha1: don't clean local variable i on exit

In this place value of variable i is constant and predictable. No point
to clean it. Furthermore, since the value is not used after assignment,
compiler will optimize and remove the assignment.

This patch fixes a warning from the clang static analyzer.

src/sha1.c

@@ -234,7 +234,6 @@ void crypto_SHA1_Final(SHA1_CTX* context, uint8_t* digest)
     }
     
     /* Wipe variables */
-    i = 0;
     memset(context->buffer, 0, 64);
     memset(context->state, 0, 20);
     memset(context->count, 0, 8);