hw/i386/isapc.c: replace rom_memory with system_memory

Now that we can guarantee the isapc machine will never have a PCI bus, any
instances of rom_memory can be replaced by system_memory and rom_memory
removed completely.

Signed-off-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20250828111057.468712-20-mark.caveayland@nutanix.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/i386/isapc.c

@@ -38,7 +38,6 @@ static void pc_init_isa(MachineState *machine)
     uint32_t irq;
     GSIState *gsi_state;
     MemoryRegion *ram_memory;
-    MemoryRegion *rom_memory = system_memory;
     DriveInfo *hd[MAX_IDE_BUS * MAX_IDE_DEVS];
     int i;
 
@@ -94,7 +93,7 @@ static void pc_init_isa(MachineState *machine)
 
     /* allocate ram and load rom/bios */
     if (!xen_enabled()) {
-        pc_memory_init(pcms, system_memory, rom_memory, 0);
+        pc_memory_init(pcms, system_memory, system_memory, 0);
     } else {
         assert(machine->ram_size == x86ms->below_4g_mem_size +
                                     x86ms->above_4g_mem_size);