Remove faulty cyrix special register read access

Patch from davefiddes. Original commit message :

GCC 10.1 reports this line as being logically always false so will never run.
On inspection it may be that this was intended to be:

   if ((cyrix_addr & 0xf0) == 0xc0) return 0xff;

According to the IBM 6X86MX Microprocessor Databook
(http://datasheets.chipdb.org/IBM/x86/6x86MX/mx_full.pdf) this would correspond
to the Address Region 0 - 3 registers but omit Address Region 4 - 7 registers
which doesn't seem to make a lot of sense.

Delete the code as the default response from the function is to return 0xff
and this is what its behviour up until now has been.

src/cpu.c

@@ -1747,7 +1747,6 @@ uint8_t cyrix_read(uint16_t addr, void *priv)
                         case 0xfe: return models[model].cpu[cpu_manufacturer].cpus[cpu].cyrix_id & 0xff;
                         case 0xff: return models[model].cpu[cpu_manufacturer].cpus[cpu].cyrix_id >> 8;
                 }
-                if ((cyrix_addr & ~0xf0) == 0xc0) return 0xff;
                 if (cyrix_addr == 0x20 && models[model].cpu[cpu_manufacturer].cpus[cpu].cpu_type == CPU_Cx5x86) return 0xff;
         }
         return 0xff;