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/testData/moduleProtoBuf/experimental/main.kt

@@ -8,5 +8,5 @@ class B {
     annotation class C
 }
 
-@Experimental(Experimental.Level.ERROR, [Experimental.Impact.COMPILATION])
+@Experimental(Experimental.Level.ERROR)
 annotation class D