[clang] Prune 'IsOMPStructuredBlock' Stmt bit

As discussed in https://reviews.llvm.org/D59214#1916596
and in some other reviews dealing with FPenv,
bits in Stmt are scarce, and i got so burnout with D59214
and https://bugs.llvm.org/show_bug.cgi?id=40563 specifically
that i never actually followed up with the usages for this bit.

So let's unhoard it, at least for now?
diff --git a/clang/lib/Serialization/ASTReaderStmt.cpp b/clang/lib/Serialization/ASTReaderStmt.cpp
index 1a3e299..4efd337 100644
--- a/clang/lib/Serialization/ASTReaderStmt.cpp
+++ b/clang/lib/Serialization/ASTReaderStmt.cpp
@@ -102,7 +102,7 @@
 
     /// The number of record fields required for the Stmt class
     /// itself.
-    static const unsigned NumStmtFields = 1;
+    static const unsigned NumStmtFields = 0;
 
     /// The number of record fields required for the Expr class
     /// itself.
@@ -138,7 +138,6 @@
 }
 
 void ASTStmtReader::VisitStmt(Stmt *S) {
-  S->setIsOMPStructuredBlock(Record.readInt());
   assert(Record.getIdx() == NumStmtFields && "Incorrect statement field count");
 }