Set multiselect to false in EnumeratedStringProperty

[peterfpeterson]

By setting isMultipleSelectionAllowed() to always return true, the correct GUI widget is displayed. cppcheck pointed out that this is the implementation in the parent and requested that I remove the useless override. Also, remove as many imports as possible with the code still compiling and operating.

This is a follow on to #38360. There is still a remaining issue with how the property type is rendered in the user docs.

There is no associated issue, but it is associated with EWM7925

Further detail of work

To test:

1. Start mantidworkbench
2. Open the dialog for Rebin
3. See that it renders how it used to

This does not require release notes because it fixes an issue introduced during the development cycle.

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[rboston628]

Dropdown works in Rebin

Dropdown works in CalculateDIFC

Dropdown works in AddSampleLog