Drop "Partial_Invis" compat handling
There's compat mode handling of "Partial_Invis" unit type flag in ruleset.c. That flag was not part of 3.1 ruleset format any more, so drop the compatibility code from master.
There's compat mode handling of "Partial_Invis" unit type flag in ruleset.c. That flag was not part of 3.1 ruleset format any more, so drop the compatibility code from master.