Only support pregenerated voice file for releases.
The voice file installed needs to match the Rockbox version installed. If not
the voice might not match the actual entries.
Rockbox Utility always was sloppy on this and installed the latest archived
(previously "daily") builds voice file for the latest development (previously
"current") build. Archived builds aren't supported anymore in Rockbox Utility,
so remove support for installing archived voice files as well.
There are no pregenerated voice files for development versions provided, and
Rockbox Utility can generate voice files anyway. Also, those files are only
provided and most users seem to prefer to use the voices they have installed on
their PC.
Change-Id: I17817da8a14dc5f8b0ef208579d8d75fa86ab610