Fix getting tank information from libdivecomputer

Silly type made us always get tank 0. So tanks 1 through n where wrong
(and repeating tank 0 instead).

Reported-by: Jef Driesen <jef@libdivecomputer.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Remove leftover code for directly parsing Cobalt tank sizes

This should have been ripped out as part of commit 4be760463422 ("Use
libdivecomputer tank size when available").

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>