InternalTrees: don't keep token cache per tree

There's already a token cache per input, so this is excessive and
unnecessary. First introduced in #3484.

scalameta/trees/shared/src/main/scala/scala/meta/internal/trees/InternalTrees.scala

@@ -66,13 +66,7 @@ trait InternalTree extends Product {
   }
 
   def tokenizeFor(dialect: Dialect): Tokens =
-    if (origin.dialectOpt.contains(dialect)) tokensOpt.get else lookupOrTokenizeFor(dialect)
-
-  private val tokenCache: mutable.Map[Dialect, Tokens] =
-    Compat.newMutableMap[Dialect, Tokens]
-
-  private def lookupOrTokenizeFor(dialect: Dialect): Tokens =
-    tokenCache.getOrElseUpdate(dialect, tokenizeForDialect(dialect))
+    if (origin.dialectOpt.contains(dialect)) tokensOpt.get else tokenizeForDialect(dialect)
 
   private lazy val tokensOpt: Option[Tokens] =
     origin match {