[llvm-exegesis] Remove getter for RegNameToRegNo mapping

This patch removes the getter for the mentioned mapping. This was only
kept around to keep things in sync for some downstream codebases (that
didn't even end up needing it), so removing it now that it is not needed
anymore.

Author: boomanaiden154

llvm/tools/llvm-exegesis/lib/LlvmState.h

@@ -76,14 +76,6 @@ class LLVMState {
     return *OpcodeNameToOpcodeIdxMapping;
   };
 
-  // TODO(boomanaiden154): We are keeping this getter around to enable internal
-  // migration to getRegisterNumberFromName. Once that is complete and
-  // the changes have been pulled, we can remove this.
-  const DenseMap<StringRef, MCRegister> &getRegNameToRegNoMapping() const {
-    assert(RegNameToRegNoMapping);
-    return *RegNameToRegNoMapping;
-  }
-
   std::optional<MCRegister>
   getRegisterNumberFromName(StringRef RegisterName) const;