[RISCV] Remove unused flags from FeatureKind in TargetParser.h. NFC

[craig.topper]

Summary

These flags aren't used and we shouldn't add more flags for new
ratified extensions. So clear out the unused flags to avoid any
confusion.

[khchen]

LGTM, thanks!

llvm/include/llvm/Support/TargetParser.h

 enum CPUKind : unsigned {
 #define PROC(ENUM, NAME, FEATURES, DEFAULT_MARCH) CK_##ENUM,
 #include "RISCVTargetParser.def"
 };
 
 enum FeatureKind : unsigned {
   FK_INVALID = 0,
   FK_NONE = 1,
-  FK_STDEXTM = 1 << 2,
-  FK_STDEXTA = 1 << 3,
-  FK_STDEXTF = 1 << 4,
-  FK_STDEXTD = 1 << 5,
-  FK_STDEXTC = 1 << 6,
-  FK_64BIT = 1 << 7,
+  FK_64BIT = 1 << 2,
 };
 
 bool checkCPUKind(CPUKind Kind, bool IsRV64);
 bool checkTuneCPUKind(CPUKind Kind, bool IsRV64);
 CPUKind parseCPUKind(StringRef CPU);
 CPUKind parseTuneCPUKind(StringRef CPU, bool IsRV64);
 StringRef getMArchFromMcpu(StringRef CPU);
 void fillValidCPUArchList(SmallVectorImpl<StringRef> &Values, bool IsRV64);