style: remove extra const keyword from interfaces

Const variables in prototypes don't add much value, but make the code
larger and redundant. Remove these const keywords.
Note, this doesn't apply to pointers to const memory.

src/util.c

@@ -38,7 +38,7 @@
  *
  *  @return a new allocates string with the same data as s or NULL on error
  */
-char *xmpp_strdup(const xmpp_ctx_t *const ctx, const char *const s)
+char *xmpp_strdup(const xmpp_ctx_t *ctx, const char *s)
 {
     size_t len;
     char *copy;
@@ -135,7 +135,7 @@ uint64_t time_elapsed(uint64_t t1, uint64_t t2)
  *
  *  @param conn a Strophe connection object
  */
-void disconnect_mem_error(xmpp_conn_t *const conn)
+void disconnect_mem_error(xmpp_conn_t *conn)
 {
     xmpp_error(conn->ctx, "xmpp", "Memory allocation error");
     xmpp_disconnect(conn);