Use libdivecomputer tank size when available

If the gasmix of a tank is not the matching gasmix in the list of
gasmixes, issue a warning (as so far we assume those are always in sync).

This patch removes our own parsing of the sizes because Subsurface 4.3
will require libdivecomputer 0.5 so this should be enabled by the time the
next release comes out, so let's just drop the redundant code.

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