stop overwriting value of emult

[clmould]

Description

Closes #1594
Stops value of emult being overwritten with hard-coded value in hcpb.py

Checklist

I confirm that I have completed the following checks:

• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[mkovari]

Does this change the results of any of the tests?

[clmould]

@mkovari I don't think any test results have changed - the tests all still pass on the CI and I also ran the tests locally to check the integration tests (which are currently not being tested in the CI) and none have failed

[mkovari]

The large tomakak regression test (regression-5-percent-py38) does run the CCFE hcpb model where the deleted code is (emult=1.269), but since the default is also 1.269 it makes no difference.