arch-x86: Delete some unused register related constants.

Change-Id: Id5305a863675061b4afb27c71b329180605381b5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39677
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/x86/registers.hh

@@ -83,14 +83,6 @@ const int NumVecPredRegs = 1;  // Not applicable to x86
 //There is no such register in X86
 const int ZeroReg = NUM_INTREGS;
 const int StackPointerReg = INTREG_RSP;
-//X86 doesn't seem to have a link register
-const int ReturnAddressReg = 0;
-const int ReturnValueReg = INTREG_RAX;
-const int FramePointerReg = INTREG_RBP;
-
-// Some OS syscalls use a second register (rdx) to return a second
-// value
-const int SyscallPseudoReturnReg = INTREG_RDX;
 
 // Not applicable to x86
 using VecElem = ::DummyVecElem;