Cop0 unused functions do not actually pose a threat (as per manual). They don't do anything, so shall we.

src/backend/core/registers/Cop0.cpp

@@ -455,7 +455,7 @@ void Cop0::decode(const Instruction instr) {
             eret();
             break;
         default:
-            panic("Unimplemented COP0 function {} ({:08X}) ({:016X})", instr.cop_funct(), u32(instr), regs.oldPC);
+            break;
         }
         break;
     default: