Variable deletion with bridges

All bridges in MOI are affine transformations except the QuadtoSOC bridge.
As deletion commutes with affine transformations, except for the QuadtoSOC bridge, there should be no issue just deleting the variable in the bridged solver model.
Deleting a variable part of a quadratic term of a quadratic function bridged by the QuadtoSOC bridged might be an issue though.
We should check that and add tests (with quadratic objective and quadratic constraints).

See https://github.com/jump-dev/MathOptInterface.jl/pull/2150/files#r1173436839