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/useExperimentalTargets.kt

@@ -1,18 +1,16 @@
 // !API_VERSION: 1.3
 // !DIAGNOSTICS: -UNUSED_PARAMETER
-// MODULE: api
 // FILE: api.kt
 
 package api
 
-@Experimental(Experimental.Level.WARNING, [Experimental.Impact.COMPILATION])
+@Experimental(Experimental.Level.WARNING)
 @Target(AnnotationTarget.CLASS)
 annotation class E
 
 @E
 open class Foo(val s: String = "")
 
-// MODULE: usage(api)
 // FILE: usage.kt
 
 import api.*