[FIR] Get rid of FirExtension.key

In reality plugin key was used only in two ways:
- found corresponding generator for member scope of generated class
- pass it to declaration origin so it can be read in backend IR plugin

Because of first usage plugin key was required to be an object, which
  reduces functionality of second usage, because there was no way to
  pass any additional info via key to IR (in future this usecase will be
  covered with IR declaration attributes, but right now we don't have
  them)

So now generator which created some class is now saved in its attributes
  and plugins can use any keys they want. And since there is no other
  usages of `FirExtension.key` it removed from FirExtension API at all

compiler/fir/tree/src/org/jetbrains/kotlin/fir/extensions/FirExtension.kt

@@ -6,7 +6,6 @@
 package org.jetbrains.kotlin.fir.extensions
 
 import org.jetbrains.kotlin.fir.FirSession
-import org.jetbrains.kotlin.fir.declarations.FirPluginKey
 import org.jetbrains.kotlin.fir.extensions.predicate.DeclarationPredicate
 import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.name.Name
@@ -24,8 +23,6 @@ typealias AnnotationFqn = FqName
 abstract class FirExtension(val session: FirSession) {
     abstract val name: FirExtensionPointName
 
-    abstract val key: FirPluginKey
-
     abstract val extensionType: KClass<out FirExtension>
 
     fun interface Factory<out P : FirExtension> {