Revert double-assignment mock prover check

Revert the check introduced in
#129 to detect double
assignments with different values, because it breaks some tests in the zkevm
project.

There's a legitimate use case of double assignment with different values, which
is overwriting cells in order to perform negative tests (tests with bad witness
that should not pass the constraints).

Also in the EVM Circuit from the zkevm project we "abuse" the assignment of
cells as a cache: sometimes we assign some cells with a guess value, and later
on we reassign with the correct value.

I believe this check is interesting to have, so we could think of ways to add
it back as an optional feature.

Author: ed255

halo2_proofs/src/dev.rs

@@ -461,12 +461,7 @@ impl<F: Field> Assignment<F> for MockProver<F> {
                     .get_mut(column.index())
                     .and_then(|v| v.get_mut(row))
                     .expect("bounds failure");
-                if let CellValue::Assigned(value) = value {
-                    // Inconsistent assignment between different phases.
-                    assert_eq!(value, &to, "value={:?}, to={:?}", value, &to);
-                } else {
-                    *value = CellValue::Assigned(to);
-                }
+                *value = CellValue::Assigned(to);
             }
             Err(err) => {
                 // Propagate `assign` error if the column is in current phase.