gluon-autoupdater: remove dead code

Recently the autoupdater contained a case statement to distinguish
between TP-Link WDR3600 and TP-Link WDR4300. This was left over from
previous development versions of autoupdater that relied on the board
name instead of the model. It is thus no longer needed and be removed
safely.
gluon-autoupdater: improve manifest.sample

Makes manifest.sample use the model instead of the board name as that is
what the script actually expects. Also, a sample signature is added.