Remove Set defaults buttons from Tool's Python and Julia settings

[soininen]

We now always use the global interpreter/executable from application settings if none is explicitly set in Tool Specification.

Re spine-tools/Spine-Toolbox#2446

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass