Simplify usages of IncrementalPackageFragment and related code

Since its scope is now empty, no descriptor now has a container that is an
instance of IncrementalPackageFragment -> a lot of code is not needed anymore

build-common/src/org/jetbrains/kotlin/incremental/IncrementalCacheImpl.kt

@@ -321,7 +321,7 @@ open class IncrementalCacheImpl<Target>(
     override fun getObsoletePackageParts(): Collection<String> {
         val obsoletePackageParts =
                 dirtyOutputClassesMap.getDirtyOutputClasses().filter { packagePartMap.isPackagePart(JvmClassName.byInternalName(it)) }
-        debugLog("Obsolete package parts: ${obsoletePackageParts}")
+        debugLog("Obsolete package parts: $obsoletePackageParts")
         return obsoletePackageParts
     }
 
@@ -346,10 +346,6 @@ open class IncrementalCacheImpl<Target>(
         return partNames.filter { !dirtyOutputClassesMap.isDirty(it) }
     }
 
-    override fun getMultifileFacade(partInternalName: String): String? {
-        return partToMultifileFacade.get(partInternalName)
-    }
-
     override fun getModuleMappingData(): ByteArray? {
         return protoMap[JvmClassName.byInternalName(MODULE_MAPPING_FILE_NAME)]?.bytes
     }