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/context.rs

@@ -353,14 +353,12 @@ pub unsafe fn dc_get_info(context: &Context) -> *mut libc::c_char {
         context,
         "SELECT COUNT(*) FROM keypairs;",
         rusqlite::NO_PARAMS,
-        0,
     );
 
     let pub_key_cnt: Option<isize> = context.sql.query_row_col(
         context,
         "SELECT COUNT(*) FROM acpeerstates;",
         rusqlite::NO_PARAMS,
-        0,
     );
 
     let fingerprint_str = if let Some(key) = Key::from_self_public(context, &l2.addr, &context.sql)