Revert "completely disable JuMP bridge constraints"
This reverts commit e99747f6. Apparently some internal JuMP logic depends on the bridges, preventing e.g. single-variable objective functions from working with GLPK (and maybe other solvers). Dodging that is easy, e.g. you can use `@objective(m, Max, x+0)` instead of one that contains just `x`, but kludges like this should not be required in the code.