[BitcodeReader] Remove unnecessary argument defaults (NFC)

This is an internal method that is always called with all arguments.

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -596,8 +596,8 @@ public:
   /// Main interface to parsing a bitcode buffer.
   /// \returns true if an error occurred.
   Error parseBitcodeInto(
-      Module *M, bool ShouldLazyLoadMetadata = false, bool IsImporting = false,
-      DataLayoutCallbackTy DataLayoutCallback = [](StringRef) { return None; });
+      Module *M, bool ShouldLazyLoadMetadata, bool IsImporting,
+      DataLayoutCallbackTy DataLayoutCallback);
 
   static uint64_t decodeSignRotatedValue(uint64_t V);