[vm, arm64] Remove dead load from Mint unboxing.
[vm, arm64] Remove dead load from Mint unboxing.

TEST=ci
Change-Id: Icf0ff5b9b822df9184533fe7163965313b4ac1df
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/181962
Reviewed-by: Liam Appelbe <78c2029e58e395825ba7228e218c841bb436a9e9@google.com>
Commit-Queue: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk