Remove intellij plugin (#1510)

This plugin is being moved to pkl-project-commons. 

Closes #1491

See https://github.com/apple/pkl-project-commons/pull/75

pkl-internal-intellij-plugin/pkl-internal-intellij-plugin.gradle.kts

@@ -1,42 +0,0 @@
-/*
- * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
- *
- * 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
- *
- *     https://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.
- */
-plugins {
-  pklAllProjects
-  kotlin("jvm")
-  alias(libs.plugins.intellij)
-}
-
-repositories {
-  mavenCentral()
-  intellijPlatform { defaultRepositories() }
-}
-
-dependencies {
-  intellijPlatform {
-    create("IC", libs.versions.intellij.get())
-    bundledPlugin("com.intellij.java")
-    bundledPlugin("org.jetbrains.plugins.gradle")
-    bundledPlugin("JUnit")
-  }
-}
-
-spotless {
-  kotlin {
-    ktfmt(libs.versions.ktfmt.get()).googleStyle()
-    target("src/*/kotlin/**/*.kt")
-    licenseHeaderFile(rootProject.file("buildSrc/src/main/resources/license-header.star-block.txt"))
-  }
-}