chore!: Remove unused Shl and Shr from Brillig IR

[kevaundray]

Description

This was spotted by Facundo.

These are not being used in the IR and so have been removed, if you need this, one should instead use Multiplication or Signed/UnsignedDiv

Problem*

Resolves

Summary*

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [Exceptional Case] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[TomAFrench]

We need to be do this over in aztec-packages as it's a serialisation change.

[kevaundray]

We need to be do this over in aztec-packages as it's a serialisation change.

Ah forgot

[TomAFrench]

Yeah, ideally our backend deserialiser should be robust enough to just ignore brillig entirely. #3907 would help on this.