Fix Tachyon detector and TTV not being deleted if exploding inside something

Fixes #2722 (Shoving a TTV in something will let it survive it's own explosion)

    code/game/objects/items/weapons/tanks/tanks.dm : 214..217

Fixes #2977 (Tachyon detector doesn't show true size)

    code/game/objects/items/weapons/tanks/tanks.dm : 208

(These bugfixes are from pull 3417 by Alek2ander , I'm just separating the bugfixes in smaller PRs)
To be honest I don't know how to make a more elegant solution for this problem myself. If this is really too ugly, I can always remove that part from the PR and make it only about the Tachyon detector fix so that the PR gets accepted..