Revert "[Bridges] use dual of equality in slack bridge (#2508)"

[odow]

This reverts commit 6d9378acf11653a1cb2ac6c969f388fa4be59f1e.

x-ref #2513

• [x] https://github.com/jump-dev/MathOptInterface.jl/actions/workflows/solver-tests.yml

[odow]

@blegat I didn't really follow the details. It looks like there is a factor of 2 slipping in, likely because the PSD matrices have symmetry?