coverage: Remove some old low-value unit tests for graph traversal

These tests might have originally been useful as an implementation aid, but now
they don't provide enough value to justify the burden of updating them as the
underlying code changes.

The code they test is still exercised by the main end-to-end coverage tests.

compiler/rustc_mir_transform/src/coverage/counters.rs

@@ -168,11 +168,6 @@ impl CoverageCounters {
         self.counter_increment_sites.len()
     }
 
-    #[cfg(test)]
-    pub(super) fn num_expressions(&self) -> usize {
-        self.expressions.len()
-    }
-
     fn set_bcb_counter(&mut self, bcb: BasicCoverageBlock, counter_kind: BcbCounter) -> BcbCounter {
         if let Some(replaced) = self.bcb_counters[bcb].replace(counter_kind) {
             bug!(