Drop version definition in `overlay.mumblelay`

[Kissaki]

The file overlay.mumblelay contains a version string at https://github.com/mumble-voip/mumble-theme/blob/9c68c1453837b82f69e93d7df2dbdd086d96f27d/overlay.mumblelay#L2.

This may be a technical necessity, but it is very bad architecture because now the Mumble version depends on the themes layout file.

It makes version bumps in Mumble more complex and error prone. https://github.com/mumble-voip/mumble/pull/3799 https://github.com/mumble-voip/mumble/issues/3761

If at all possible the theme must not contain version specific code like this.

@xPoke Is the assumption correct that this is a layout file replacing the existing one for layout changes. And the version number being in there is a side effect because we override the entire file?

So the way to fix this would then to look for a way in Mumble itself to clear it from the layout file? (What is the file used for and how?)