Use SuppressibleTreePathScanner instead of rolling our own.

This is already covered by tests, such as `negativeCases_suppressionForMethodTreeBased()`.

(`SuppressibleTreePathScanner` might actually correct some edge cases somewhere, even.)

PiperOrigin-RevId: 580260707

core/src/main/java/com/google/errorprone/bugpatterns/nullness/ReturnMissingNullable.java

@@ -62,7 +62,6 @@
 import com.sun.source.tree.MethodTree;
 import com.sun.source.tree.ReturnTree;
 import com.sun.source.tree.StatementTree;
-import com.sun.source.tree.Tree;
 import com.sun.source.tree.VariableTree;
 import com.sun.source.util.TreePathScanner;
 import com.sun.tools.javac.code.Symbol;
@@ -230,7 +229,7 @@ void doVisitVariable(VariableTree tree) {
     }.scan(tree, null);
     ImmutableSet<VarSymbol> definitelyNullVars = definitelyNullVarsBuilder.build();
 
-    new TreePathScanner<Void, Void>() {
+    new SuppressibleTreePathScanner<Void, Void>(stateForCompilationUnit) {
       @Override
       public Void visitBlock(BlockTree block, Void unused) {
         for (StatementTree statement : block.getStatements()) {
@@ -398,14 +397,6 @@ && methodCanBeOverridden(method)) {
           }
         }
       }
-
-      @Override
-      public Void scan(Tree tree, Void unused) {
-        if (isSuppressed(tree, stateForCompilationUnit)) {
-          return null;
-        }
-        return super.scan(tree, unused);
-      }
     }.scan(tree, null);
 
     return NO_MATCH; // Any reports were made through state.reportMatch.