Rollup merge of #123439 - Zalathar:constants, r=oli-obk

coverage: Remove useless constants

After #122972 and #123419, these constants don't serve any useful purpose, so get rid of them.

`@rustbot` label +A-code-coverage

compiler/rustc_mir_transform/src/coverage/query.rs

@@ -59,7 +59,7 @@ fn coverage_ids_info<'tcx>(
             _ => None,
         })
         .max()
-        .unwrap_or(CounterId::START);
+        .unwrap_or(CounterId::ZERO);
 
     CoverageIdsInfo { max_counter_id }
 }