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/cli/jvm/experimentalAndUseExperimentalWithSameAnnotation.kt

@@ -1,4 +1,4 @@
 package org.test
 
-@Experimental(Experimental.Level.ERROR, [Experimental.Impact.COMPILATION])
+@Experimental
 annotation class ExperimentalAPI