BlueprintBuilder: remove SledEditor::baseboard_id()

[jgallagher]

SledEditor was storing each sled's baseboard ID, but:

• it's only available via PlanningInput, and putting it directly into the blueprint is difficult for mostly-irrelevant reasons
• there was only one caller of SledEditor::baseboard_id(): sled_ensure_mupdate_override() (to match up sled ID <-> baseboard ID)
• sled_ensure_mupdate_override() itself only has one caller, the planner, which already knows the baseboard ID of the sled it's ensuring

So this PR removes SledEditor::baseboard_id() entirely, and has the planner pass the baseboard ID in. (On the update watercooler today we also mentioned the possibility of a separate BlueprintBuilder::clear_pending_mgs_updates(baseboard_id) method, but that's tricky to implement because some nontrivial logic for "should we clear pending MGS updates" is nested inside SledEditor itself.)

[sunshowers]

nice!