AI never builds sea-only workers. As we don't have worker selection mechanism
fine-grained enough to select workers based on where they are needed, disallowing
sea-only workers completely is better than ending up using only them in some rulesets,
meaning no worker would work land tiles. Amphibious workers working both land and
sea are still supported.

See patch #4610


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24767 a0f10bec-cc02-0410-94fc-a9cfff90b4cd