Implicit 'Build Requirements' from 'Builders' are not removed when changing 'Builders', causes builds to not execute

When creating/copying a plan, there are implicit build requirements derived from the selected builder. These requirements can not be managed by the user, but are added and deleted on their behalf by Bamboo when selecting a builder.

In 2.6 build 1901 - 27 May 10, when one changed the builder for a plan; the old builder requirement is not removed from the plan:

This causes the build to no be able to schedule on desired agents, leaving it in the build queue.

Once saved, the requirements can not be removed; cloning the build plan doesn't work around this issue.

This will only affect plans that change builders.