Update the tile info after a city is removed because infrastructures may have
changed.

From a report by Emmet Hikory (persia@gna)

See gna bug #22074

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25263 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
When creating a city, hide the units on the tile to non-allied players. When
removing a city, reveal units on the tile to non-allied players.

Additionally remove the hack in server_remove_unit() to prevent duplicates of
unit removals.

See gna bug #22222

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25266 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
When removing a unit from the game (server side), clear the vision before
sending unit remove. Else, we might duplicate the PACKET_UNIT_REMOVE if we lose
vision of the unit tile.

From a report by Jacob Nevins (jtn@gna)

See gna bug #22138

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25268 a0f10bec-cc02-0410-94fc-a9cfff90b4cd