[MLIR] [Arith] [NFC] Remove switch statement default label due to warning

We are getting compiler warning because default label is excessive. Removing it for now.

[[ http://lab.llvm.org:8011/#/builders/61/builds/22782/steps/5/logs/warnings__2_ | view on buildbot ]]

```
/vol/worker/mlir-nvidia/mlir-nvidia/llvm.src/mlir/lib/Dialect/Arithmetic/IR/ArithmeticOps.cpp:1597:9: warning: default label in switch which covers all enumeration values [-Wcovered-switch-default]
```

Related to: [[ https://reviews.llvm.org/D117257 | D117257 ]]

Reviewed By: stella.stamenova

Differential Revision: https://reviews.llvm.org/D120730