Remove NoFunctionalReturnType.

I think the higher-value findings from this would be caught by UnnecessaryLambda.

PiperOrigin-RevId: 438838411

Author: graememorgan

core/src/main/java/com/google/errorprone/bugpatterns/NoFunctionalReturnType.java

@@ -238,7 +238,6 @@
 import com.google.errorprone.bugpatterns.NegativeCharLiteral;
 import com.google.errorprone.bugpatterns.NestedInstanceOfConditions;
 import com.google.errorprone.bugpatterns.NoAllocationChecker;
-import com.google.errorprone.bugpatterns.NoFunctionalReturnType;
 import com.google.errorprone.bugpatterns.NonAtomicVolatileUpdate;
 import com.google.errorprone.bugpatterns.NonCanonicalStaticImport;
 import com.google.errorprone.bugpatterns.NonCanonicalStaticMemberImport;
@@ -1065,7 +1064,6 @@ public static ScannerSupplier errorChecks() {
           MultiVariableDeclaration.class,
           MultipleTopLevelClasses.class,
           NoAllocationChecker.class,
-          NoFunctionalReturnType.class,
           NonCanonicalStaticMemberImport.class,
           PackageLocation.class,
           ParameterComment.class,