Remove faulty assignment (kernel to variable)

[freddieknets]

Description

Small bugfix in slicer: slicing kernel was assigned to an internal variable (which creates faulty behaviour when copying an instance).
All tests pass, as well as all xwakes tests.

Checklist

Mandatory:

• I have added tests to cover my changes
• All the tests are passing, including my new ones
• I described my changes in this PR description

Optional:

• The code I wrote follows good style practices (see PEP 8 and PEP 20).
• I have updated the docs in relation to my changes, if applicable
• I have tested also GPU contexts