coverage: Remove unnecessary `bcb_successors`

Given that we directly access the graph predecessors/successors in so many
other places, and sometimes must do so to satisfy the borrow checker, there is
little value in having this trivial helper method.

compiler/rustc_mir_transform/src/coverage/counters.rs

@@ -323,8 +323,7 @@ impl<'a> MakeBcbCounters<'a> {
         }
 
         // Determine the set of out-edges that don't yet have edge counters.
-        let candidate_successors = self
-            .bcb_successors(from_bcb)
+        let candidate_successors = self.basic_coverage_blocks.successors[from_bcb]
             .iter()
             .copied()
             .filter(|&to_bcb| self.edge_has_no_counter(from_bcb, to_bcb))
@@ -502,11 +501,6 @@ impl<'a> MakeBcbCounters<'a> {
         None
     }
 
-    #[inline]
-    fn bcb_successors(&self, bcb: BasicCoverageBlock) -> &[BasicCoverageBlock] {
-        &self.basic_coverage_blocks.successors[bcb]
-    }
-
     #[inline]
     fn edge_has_no_counter(
         &self,