[MC] Remove an unnecessary cast (NFC) (llvm#146251)

Author: kazutakahirata

llvm/lib/MC/MCDwarf.cpp

@@ -1879,7 +1879,7 @@ struct CIEKey {
   unsigned LsdaEncoding = -1;
   bool IsSignalFrame = false;
   bool IsSimple = false;
-  unsigned RAReg = static_cast<unsigned>(UINT_MAX);
+  unsigned RAReg = UINT_MAX;
   bool IsBKeyFrame = false;
   bool IsMTETaggedFrame = false;
 };