[flang] Remove runtime check from OpenFile::Close()

In error cases it is possible to CLOSE a unit that has not
been successfully connected, so don't crash when the file descriptor
is negative.

Differential Revision: https://reviews.llvm.org/D115165

Author: klausler

flang/runtime/file.cpp

@@ -157,7 +157,6 @@ void OpenFile::Predefine(int fd) {
 }
 
 void OpenFile::Close(CloseStatus status, IoErrorHandler &handler) {
-  CheckOpen(handler);
   pending_.reset();
   knownSize_.reset();
   switch (status) {