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/dc_imex.rs

@@ -606,7 +606,7 @@ unsafe fn import_backup(context: &Context, backup_to_import: *const libc::c_char
 
     let total_files_cnt = context
         .sql
-        .query_row_col::<_, isize>(context, "SELECT COUNT(*) FROM backup_blobs;", params![], 0)
+        .query_row_col::<_, isize>(context, "SELECT COUNT(*) FROM backup_blobs;", params![])
         .unwrap_or_default() as usize;
     info!(
         context,