Drop ExternalDeclarationsProvider

It was added for Android extensions, but now another mechanism is used there
(PackageFragmentProviderExtension), and there were no other implementations of
ExternalDeclarationsProvider in the project

compiler/frontend/src/org/jetbrains/kotlin/extensions/ExternalDeclarationsProvider.kt

@@ -1,30 +0,0 @@
-/*
- * Copyright 2010-2015 JetBrains s.r.o.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package org.jetbrains.kotlin.extensions
-
-import org.jetbrains.kotlin.extensions.ProjectExtensionDescriptor
-import org.jetbrains.kotlin.analyzer.ModuleInfo
-import org.jetbrains.kotlin.psi.KtFile
-
-interface ExternalDeclarationsProvider {
-    companion object : ProjectExtensionDescriptor<ExternalDeclarationsProvider>(
-            "org.jetbrains.kotlin.externalDeclarationsProvider",
-            ExternalDeclarationsProvider::class.java
-    )
-
-    fun getExternalDeclarations(moduleInfo: ModuleInfo?): Collection<KtFile>
-}