mia: remove support for loading ".smd" extension

This incorrectly implies that we support the old .smd rom format that used to be standard prior to .bin/.md;

This format is an interleaved format designed for the Super Magic Drive copier and ares does not support it

To prevent confusing users who may sill have legacy .smd files around, I'm removing the extension from mia.

mia/medium/mega-drive.cpp

@@ -1,6 +1,6 @@
 struct MegaDrive : Cartridge {
   auto name() -> string override { return "Mega Drive"; }
-  auto extensions() -> vector<string> override { return {"md", "smd", "gen", "bin"}; }
+  auto extensions() -> vector<string> override { return {"md", "gen", "bin"}; }
   auto load(string location) -> bool override;
   auto save(string location) -> bool override;
   auto analyze(vector<u8>& rom) -> string;