arch-x86: Stop printing a source operand for the monitor instruction.

Like CPUID, all operands for this instruction are implicit and are not
written out when putting it into assembly. Stop printing a source
operand when generating disassembly in gem5.

Change-Id: I95898afdd9101ad393b3aace99536db602752a64
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49225
Maintainer: Gabe Black <gabe.black@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>

src/arch/x86/isa/formats/monitor_mwait.isa

@@ -27,8 +27,6 @@ output decoder {{
         std::stringstream response;
 
         printMnemonic(response, mnemonic);
-        ccprintf(response, " ");
-        printReg(response, srcRegIdx(0), machInst.opSize);
         return response.str();
     }
 }};