Disable symmetry breaker for unsat cores (#1958)

Disable symmetry breaker for unsat cores (#1958)

Currently, --check-unsat-cores fails for one of our regressions. It
turns out that the issue is due to preprocessing done by the symmetry
breaker.  The symmetry breaker does not add dependencies and thus the
generated unsat core is incomplete/wrong. This commit disables the
symmetry breaker when we are generating unsat cores (it was previously
only disabled when generating proofs). Fixes issue #1953.