Merge the BuildSchedulingInfo table into the Builds table

This simplifies the code and improves performance since it reduces
the number of joins.

src/script/hydra-update-gc-roots

@@ -112,7 +112,7 @@ keepBuild $_ foreach @buildsToKeep;
 
 # For scheduled builds, we register the derivation as a GC root.
 print STDERR "*** looking for scheduled builds\n";
-foreach my $build ($db->resultset('Builds')->search({finished => 0}, {join => 'schedulingInfo'})) {
+foreach my $build ($db->resultset('Builds')->search({finished => 0})) {
     if (isValidPath($build->drvpath)) {
         print STDERR "keeping scheduled build ", $build->id, " (",
             strftime("%Y-%m-%d %H:%M:%S", localtime($build->timestamp)), ")\n";