Remove deprecated/unused ThreadPoolTypes

[ldematte]

Only caveat: ThreadPoolType.fromType is used in transport protocol serialization.
I was thinking to not introduce a new TV, but to backport this change and add an explicit fallback for v8.last; v8.last will check if it is one of the deprecated / removed in v9 strings, and fallback (defaulting to fixed):

public class ThreadPool {
   public static class Info {
      public Info(StreamInput in) throws IOException {
            name = in.readString();
            // Here: in the backport only
+          var typeAsString = in.readString();
+          if ("fixed_auto_queue_size".equals(typeAsString) || "direct".equals(typeAsString)) {
+             type = ThreadPoolType.FIXED;
+          } else {
+             type = ThreadPoolType.fromType(typeAsString);
+          }
-           type = ThreadPoolType.fromType(in.readString());
            min = in.readInt();
            max = in.readInt();
            keepAlive = in.readOptionalTimeValue();
            queueSize = in.readOptionalWriteable(SizeValue::new);
        }

So there will be no chance v9 will ever see these ThreadPoolType strings.

[ldematte]

@elasticmachine update branch

[ldematte]

Obsolete