[CHERI-RISC-V] Stop manually handling xcheri feature

This may have been needed before ISAInfo handled custom extensions, but
now we can just defer to ISAInfo. No functional change intended

Author: arichardson

clang/lib/Basic/Targets/RISCV.cpp

@@ -437,7 +437,6 @@ bool RISCVTargetInfo::hasFeature(StringRef Feature) const {
                     .Case("32bit", !Is64Bit)
                     .Case("64bit", Is64Bit)
                     .Case("experimental", HasExperimental)
-                    .Case("xcheri", HasCheri)
                     .Default(std::nullopt);
   if (Result)
     return *Result;