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.
3 jobs for mk-revert-disable-bridges in 10 minutes and 37 seconds
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passed format #293966
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  Documentation
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