hypervisor: emulator: drop the unused gva_to_gpa hook

That function is too limiting. It doesn't consider page permissions. It
is not used, so just drop it.

Signed-off-by: Wei Liu <liuwe@microsoft.com>

hypervisor/src/arch/x86/emulator/mod.rs

@@ -750,10 +750,6 @@ mod mock_vmm {
             Ok(())
         }
 
-        fn gva_to_gpa(&self, gva: u64) -> Result<u64, PlatformError> {
-            Ok(gva)
-        }
-
         fn fetch(&self, ip: u64, instruction_bytes: &mut [u8]) -> Result<(), PlatformError> {
             let rip = self
                 .state