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.