Drop Experimental.Impact, simplify related code and tests

See https://github.com/Kotlin/KEEP/issues/95#issuecomment-383889404

Drop Experimental.changesMayBreak, Experimental.Impact, the concept of
signature/body usage, same module exemption. Make the majority of tests
single-module because there is now no difference in the checker between
usages from the same module or from another module

compiler/tests/org/jetbrains/kotlin/cli/CliTestGenerated.java

@@ -186,11 +186,6 @@ public class CliTestGenerated extends AbstractCliTest {
             runTest("compiler/testData/cli/jvm/experimentalNested.args");
         }
 
-        @TestMetadata("experimentalRuntimeScope.args")
-        public void testExperimentalRuntimeScope() throws Exception {
-            runTest("compiler/testData/cli/jvm/experimentalRuntimeScope.args");
-        }
-
         @TestMetadata("experimentalUnresolved.args")
         public void testExperimentalUnresolved() throws Exception {
             runTest("compiler/testData/cli/jvm/experimentalUnresolved.args");