[Bridges] fix deleting variable in bridged objective

[odow]

This is a bit of a weird case: if you delete a variable that was an objective, does the type of the objective change, or do we remove one of the objective rows?

I obviously thought so when writing the tests

MathOptInterface.jl/src/Test/test_multiobjective.jl

Lines 21 to 35 in 403bc58

	function test_multiobjective_vector_of_variables_delete(
	model::MOI.ModelLike,
	::Config{T},
	) where {T}
	F = MOI.VectorOfVariables
	@requires MOI.supports(model, MOI.ObjectiveFunction{F}())
	x = MOI.add_variables(model, 2)
	f = MOI.VectorOfVariables(x)
	MOI.set(model, MOI.ObjectiveSense(), MOI.MIN_SENSE)
	MOI.set(model, MOI.ObjectiveFunction{F}(), f)
	@test MOI.get(model, MOI.ObjectiveFunctionType()) == F
	@test MOI.get(model, MOI.ObjectiveFunction{F}()) == f
	MOI.delete(model, x[1])
	@test MOI.get(model, MOI.ObjectiveFunction{F}()) ==
	MOI.VectorOfVariables([x[2]])

but I missed the case that this could also be bridged.

[blegat]

This is a bit of a weird case: if you delete a variable that was an objective, does the type of the objective change, or do we remove one of the objective rows?

You remove the row, that's consistent with what happens in a VectorOfVariables-in-Nonnegatives constraint

[blegat]

Maybe create a utility function that is called with discard and isequal(vi) ?