Check if rt.jar is still necessary (#4206)

Fixes #4164

It's been around since we used to use Ammonite to compile scripts, and
IIRC was necessary to make the in-memory Scala compiler work. We no
longer go through that code path, so maybe we can get rid of it? AFAIK
we now only use the Scala compiler through Zinc which manages this stuff
itself

Author: lihaoyi

Diff for: main/api/src/mill/api/ClassLoader.scala

@@ -2,11 +2,6 @@ package mill.api
 
 import java.net.{URL, URLClassLoader}
 
-import java.nio.file.{FileAlreadyExistsException, FileSystemException}
-
-import mill.java9rtexport.Export
-import scala.util.Properties
-
 /**
  * Utilities for creating classloaders for running compiled Java/Scala code in
  * isolated classpaths.
@@ -22,10 +17,7 @@ object ClassLoader {
       sharedPrefixes: Seq[String] = Seq(),
       logger: Option[mill.api.Logger] = None
   )(implicit ctx: Ctx.Home): URLClassLoader = {
-    new URLClassLoader(
-      makeUrls(urls).toArray,
-      refinePlatformParent(parent)
-    ) {
+    new URLClassLoader(urls.toArray, refinePlatformParent(parent)) {
       override def findClass(name: String): Class[?] = {
         if (sharedPrefixes.exists(name.startsWith)) {
           logger.foreach(
@@ -73,31 +65,4 @@ object ClassLoader {
       appClassLoader.getParent()
     }
   }
-
-  private def makeUrls(urls: Seq[URL])(implicit ctx: Ctx.Home): Seq[URL] = {
-    if (java9OrAbove) {
-      val java90rtJar = ctx.home / Export.rtJarName
-      if (!os.exists(java90rtJar)) {
-        // Time between retries should go from 100 ms to around 10s, which should
-        // leave plenty of time for another process to write fully this 50+ MB file
-        val retry = Retry(
-          count = 7,
-          backoffMillis = 100,
-          filter = {
-            case (_, _: FileSystemException) if Properties.isWin => true
-            case _ => false
-          }
-        )
-        retry {
-          try os.copy(os.Path(Export.rt()), java90rtJar, createFolders = true)
-          catch {
-            case e: FileAlreadyExistsException => /* someone else already did this */
-          }
-        }
-      }
-      urls :+ java90rtJar.toIO.toURI().toURL()
-    } else {
-      urls
-    }
-  }
 }

Diff for: runner/src/mill/runner/MillMain.scala

@@ -6,7 +6,6 @@ import java.nio.file.StandardOpenOption
 import java.util.Locale
 import scala.jdk.CollectionConverters.*
 import scala.util.Properties
-import mill.java9rtexport.Export
 import mill.api.{MillException, SystemStreams, WorkspaceRoot, internal}
 import mill.bsp.{BspContext, BspServerResult}
 import mill.main.BuildInfo
@@ -195,16 +194,6 @@ object MillMain {
 
                 val threadCount = Some(maybeThreadCount.toOption.get)
 
-                if (mill.main.client.Util.isJava9OrAbove) {
-                  val rt = config.home / Export.rtJarName
-                  if (!os.exists(rt)) {
-                    streams.err.println(
-                      s"Preparing Java ${System.getProperty("java.version")} runtime; this may take a minute or two ..."
-                    )
-                    Export.rtTo(rt.toIO, false)
-                  }
-                }
-
                 val bspContext =
                   if (bspMode) Some(new BspContext(streams, bspLog, config.home)) else None