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/keyring.rs

@@ -37,7 +37,6 @@ impl<'a> Keyring<'a> {
             context,
             "SELECT private_key FROM keypairs ORDER BY addr=? DESC, is_default DESC;",
             &[self_addr.as_ref()],
-            0,
         )
         .and_then(|blob: Vec<u8>| Key::from_slice(&blob, KeyType::Private))
         .map(|key| self.add_owned(key))