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/experimentalDeprecated.kt

@@ -1,17 +1,17 @@
 package org.test
 
-@Deprecated("BinaryError", level = DeprecationLevel.ERROR)
-@Experimental(Experimental.Level.ERROR, [Experimental.Impact.RUNTIME])
-annotation class BinaryError
+@Deprecated("Error1", level = DeprecationLevel.ERROR)
+@Experimental
+annotation class Error1
 
-@Deprecated("BinaryHidden", level = DeprecationLevel.HIDDEN)
-@Experimental(Experimental.Level.ERROR, [Experimental.Impact.RUNTIME])
-annotation class BinaryHidden
+@Deprecated("Error2", level = DeprecationLevel.ERROR)
+@Experimental
+annotation class Error2
 
-@Deprecated("SourceError", level = DeprecationLevel.ERROR)
-@Experimental(Experimental.Level.ERROR, [Experimental.Impact.COMPILATION])
-annotation class SourceError
+@Deprecated("Hidden1", level = DeprecationLevel.HIDDEN)
+@Experimental
+annotation class Hidden1
 
-@Deprecated("SourceHidden", level = DeprecationLevel.HIDDEN)
-@Experimental(Experimental.Level.ERROR, [Experimental.Impact.COMPILATION])
-annotation class SourceHidden
+@Deprecated("Hidden2", level = DeprecationLevel.HIDDEN)
+@Experimental
+annotation class Hidden2