r12207: when bankrupt, roadsigns on train tracks are not removed

[Nazalassa]

Title says it all. Track gets removed, not roadsign.

Patch: https://simutrans.net/victor_18993/bankrupt-signs.patch

[victor_18993]

Fixed and integrated in r12208.
The issue was that several railway signs are implemented as

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roadsign_t, so during company bankruptcy they were being transferred to the public player while the underlying railway was removed, leaving orphaned signs behind.
Nazalassa's patch was correct as-is and has been integrated unchanged. A regression test was also added to cover the affected railway signs, normal road signs and public-player ownership cases.
Full test suite: 293/293 passed.
Thanks again, Nazalassa, for the report and the patch.