Simplify Sql.query_row_col function

Function `query_row` executes query and calls callback function to process
row returned. Function query_row_col() is special case, that provides
callback function, which returns value of one particular column of row,
ignoring others.

In all cases, that particular column was 0 (first and only column of
query result), since there is no point to select more than one column
with this function -- they are discarded anyway.

This commit removes that redundancy, removing column number argument of
query_row_col() function and adjusting call sites accordingly.

src/securejoin.rs

@@ -758,7 +758,6 @@ pub fn handle_degrade_event(context: &Context, peerstate: &Peerstate) {
                 context,
                 "SELECT id FROM contacts WHERE addr=?;",
                 params![&peerstate.addr],
-                0,
             )
             .unwrap_or_default();
         if contact_id > 0 {