[query] Remove PSubsetStruct (#15926)

It has been supplanted by other struct STypes for projections. And it's
not used at current and cannot be constructed outside of the parser. It
was never serialized in a durable fashion and so its removal is safe.


## Security Assessment
- This change cannot impact the Hail Batch instance as deployed by Broad
Institute in GCP

Author: chrisvittal

hail/hail/src/is/hail/expr/ir/Parser.scala

@@ -454,13 +454,6 @@ object IRParser {
         punctuation(it, "}")
         val fields = args.zipWithIndex.map { case ((id, t), i) => PField(id, t, i) }
         PCanonicalStruct(fields, req)
-      case "PSubsetStruct" =>
-        punctuation(it, "{")
-        val parent = ptype_expr(rgs)(it).asInstanceOf[PStruct]
-        punctuation(it, "{")
-        val args = repsepUntil(it, identifier, PunctuationToken(","), PunctuationToken("}"))
-        punctuation(it, "}")
-        PSubsetStruct(parent, args)
     }
     assert(typ.required == req)
     typ

hail/hail/src/is/hail/types/physical/PSubsetStruct.scala

@@ -25,10 +25,7 @@ abstract class PhysicalTestUtils extends Logging {
     val srcRegion = Region(pool = pool)
     val region = Region(pool = pool)
 
-    val srcAddress = sourceType match {
-      case x: PSubsetStruct => ScalaToRegionValue(srcRegion, x.ps, sourceValue)
-      case _ => ScalaToRegionValue(srcRegion, sourceType, sourceValue)
-    }
+    val srcAddress = ScalaToRegionValue(srcRegion, sourceType, sourceValue)
 
     if (interpret) {
       try {