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/diagnostics/testsWithStdLib/experimental/override.kt

@@ -1,10 +1,9 @@
 // !API_VERSION: 1.3
-// MODULE: api
 // FILE: api.kt
 
 package api
 
-@Experimental(Experimental.Level.WARNING, [Experimental.Impact.COMPILATION])
+@Experimental(Experimental.Level.WARNING)
 annotation class E
 
 open class Base {
@@ -16,7 +15,6 @@ class DerivedInSameModule : Base() {
     override fun <!EXPERIMENTAL_OVERRIDE!>foo<!>() {}
 }
 
-// MODULE: usage1(api)
 // FILE: usage-propagate.kt
 
 package usage1
@@ -35,7 +33,6 @@ class Derived2 : Base() {
     override fun foo() {}
 }
 
-// MODULE: usage2(api)
 // FILE: usage-none.kt
 
 package usage2