[clang] NFC: Remove unused FileEntry::getLastRef() (#68156)

The last usage of the deprecated `FileEntry::getLastRef()` was removed
in #67838, let's remove it entirely.

Author: jansvoboda11

clang/include/clang/Basic/FileEntry.h

@@ -426,7 +426,6 @@ class FileEntry {
 public:
   ~FileEntry();
   StringRef getName() const { return LastRef->getName(); }
-  FileEntryRef getLastRef() const { return *LastRef; }
 
   StringRef tryGetRealPathName() const { return RealPathName; }
   off_t getSize() const { return Size; }