[ASTReader] Remove assert

This assert was reportedly added to "Defensively ensure that
GetExternalDeclStmt protects itself from nested deserialization". Given
that this triggers without nested deserialization we should remove this
assert. The assert blocks swiftlang/swift#81859

rdar://153085264

Author: hnrklssn

clang/lib/Serialization/ASTReader.cpp

@@ -8035,8 +8035,6 @@ Stmt *ASTReader::GetExternalDeclStmt(uint64_t Offset) {
     Error(std::move(Err));
     return nullptr;
   }
-  assert(NumCurrentElementsDeserializing == 0 &&
-         "should not be called while already deserializing");
   Deserializing D(this);
   return ReadStmtFromStream(*Loc.F);
 }