vfs_static: Remove template byte parameter from StaticVfsFile

This converts it into a regular constructor parameter. There's no need
to make this a template parameter on the class when it functions
perfectly well as a constructor argument.

This also reduces the amount of code bloat produced by the compiler, as
it doesn't need to generate the same code for multiple different
instantiations of the same class type, but with a different fill value.

src/core/file_sys/romfs.cpp

@@ -134,7 +134,7 @@ VirtualFile CreateRomFS(VirtualDir dir) {
         return nullptr;
 
     RomFSBuildContext ctx{dir};
-    return ConcatenateFiles<0>(ctx.Build(), dir->GetName());
+    return ConcatenateFiles(0, ctx.Build(), dir->GetName());
 }
 
 } // namespace FileSys