Enable idempotency check (#164)

Trying to enable this check that was disabled because of a
non-idempotent migration.

cc @liamaharon 

- [x] Does not require a CHANGELOG entry

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: fellowship-merge-bot[bot] <151052383+fellowship-merge-bot[bot]@users.noreply.github.com>

.github/workflows/check-migrations.yml

@@ -96,14 +96,6 @@ jobs:
             echo "Enabling the spec version check since we are releasing"
           fi
 
-          # Disable idemepotency checks on Polkadot until we enact them.
-          if [[ "${{ matrix.runtime.name }}" == "polkadot" ]]; then
-            echo "Disabling the idempotency check since we are on Polkadot"
-            EXTRA_FLAGS+=" --disable-idempotency-checks"
-          else
-            echo "Enabling the idempotency check since we are not on Polkadot"
-          fi
-
           echo "Flags: $EXTRA_FLAGS"
 
           ./try-runtime \