Replace the Unbribable_Units effect with the Bribe Unit action enabler

It is possible to write the same rules using action enablers for the
"Bribe Unit" action. Since the target requirements on the Bribe Unit action
enabler now are checked in the same places as the Unbribable_Units effect no
functionality is lost by removing this redundancy.

By removing it the code won't have to take into account that the effect can
stop a bribe even if the action is enabled. It can also ignore that both
could be active at once. Another benefit from removing the now redundant
Unbribable_Units effect is that users that wish to edit a ruleset won't have
to discover both the action enabler and the effect to get full control.

All use of the Unbribable_Units effect is replaced with "Bribe Unit" action
enablers in the bundled rule sets.

See patch #4578

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24612 a0f10bec-cc02-0410-94fc-a9cfff90b4cd