Refactor Inelastic QENS FitTab

[robertapplin]

Description of work

This PR does some refactoring of the FitTab of Inelastic QENS Fitting interface. It reduces the amount of code in this FitTab by removing unused code, and moving some of the code to more testable locations, such as the FittingModel of some of the Presenters.

Part of #36390

To test:

Follow the testing instructions here

https://developer.mantidproject.org/Testing/Inelastic/QENSFittingTests.html

This does not require release notes because it is an internal change

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[github-actions]

👋 Hi, @robertapplin,

Conflicts have been detected against the base branch. Please rebase your branch against the base branch.

[sf1919]

pre-commit.ci run

[adriazalvarez]

Thanks for this neat refactoring.
I have added couple of comments.
Additionally, I have found that when Run button is pressed from any of the QENS tab with a fit function selected, an empty warning dialog appears:

Can you check if you get the same?

[robertapplin]

Thanks for spotting that weird warning, not sure how I missed it!

[adriazalvarez]

Thanks for the changes. It looks good to me.