Remove top-level SymExprs from the AST (chapel-lang#26321)

They aren't used for anything, so deleting them is reasonable.

This fixes chapel-lang#25885. The crux
of that issue is that we incorrectly identify the `R` type as "unused",
since it's generic but not used for any sub-classes and is never
instantiated. I first attempted to fix the problem by adjusting
`isUnusedClass` to respect `SymExprs` still in the tree that still point
to the class. However, subsequent code __deletes all generic type
declarations, because by that point we expect all occurrences to be
concrete__:

```Chapel
new genericType(...); // replaced with a call to concreteType.init
var x: genericType = ...; // replaced with `var x: concreteType`

// the following:
proc foo(type arg) {}
foo(genericType)
// is replaced with:
proc foo() {}
foo();
```

Deleting all generic types seems like a reasonable approach to me; the
standalone variables are a weird exception, in which the type is neither
instantiated nor removed. Moreover, since they don't modify or even
really access the variable, these standalone `SymExprs` can be safely
removed. This PR does just that.

Reviewed by @jabraham17 -- thanks!

## Testing
- [x] paratest

compiler/resolution/cleanups.cpp

@@ -124,6 +124,14 @@ static void removeUnusedFunctions() {
   }
 }
 
+static void removeTopLevelSymExprs() {
+  for_alive_in_Vec(SymExpr, se, gSymExprs) {
+    if (se->getStmtExpr() == se) {
+      se->remove();
+    }
+  }
+}
+
 static CallExpr* replaceRuntimeTypeGetField(CallExpr* call) {
   SymExpr* rt = toSymExpr(call->get(1));
 
@@ -1038,6 +1046,8 @@ void pruneResolvedTree() {
 
   removeUnusedFunctions();
 
+  removeTopLevelSymExprs();
+
   if (fRemoveUnreachableBlocks) {
     deadBlockElimination();
   }

test/types/records/generic/generic-type-standalone-mention.chpl

@@ -0,0 +1,2 @@
+record R { var arg; }
+R;