Remove unused mg_hmac_sha1()

src/sha1.h

@@ -11,6 +11,3 @@ typedef struct {
 void mg_sha1_init(mg_sha1_ctx *);
 void mg_sha1_update(mg_sha1_ctx *, const unsigned char *data, size_t len);
 void mg_sha1_final(unsigned char digest[20], mg_sha1_ctx *);
-void mg_hmac_sha1(const unsigned char *key, size_t key_len,
-                  const unsigned char *text, size_t text_len,
-                  unsigned char out[20]);