LightTool : Remove unused variables

[ericmehl]

I don't know why this was not caught during the builds for #5483, but @danieldresser found that offset is causing a warning about offset not being used in LengthHandle::setupDrag(). inspection in that same function is also not being used, so I removed it as well.

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Gaffer project's prevailing coding style and conventions.

[johnhaddon]

I already removed that and some more in #5514 which I opened yesterday - you're down as the reviewer :)

[ericmehl]

Ahhhh, those were the Mac build fixes. Moving along then, nothing to see here...