Migrate away from select2 in app/assets/javascripts/issuable/issuable_form.js

The IssuableForm#initTargetBranchDropdown method seems to be initializing select2 instances on .js-target-branch-select selectors. However, we don't seem to be rendering .js-target-branch-select anywhere in the codebase, so we might be able to get rid of this method altogether.

Internally, the method leverages the AutoWidthDropdownSelect class which would likely become unused if the method was removed. We should consider deleting app/assets/javascripts/issuable/auto_width_dropdown_select.js as well.

Implementation plan

• Remove the method after having confirmed that it's not used anymore.
• Delete app/assets/javascripts/issuable/auto_width_dropdown_select.js if it isn't used anymore.

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@pgascouvaillancourt, thanks for adding the workflowready for development label.

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marked this issue as blocking #374020 (closed)

mentioned in issue #369990

marked this issue as related to #369990

mentioned in issue #374116 (closed)

assigned to @samdbeckham

This was originally used for the target branch selector. https://gitlab.com/gitlab-org/gitlab/-/blob/d17cda1c6ba0a969e77477befad8c5fb455657b5/app/views/shared/issuable/form/_branch_chooser.html.haml#L19

The need for this was removed recently in !96620 (merged) by @thomasrandolph

I haven't found any other uses of it in the codebase form the last 5 years so I thin we can safely start removing it. I'll open an MR shortly to do that.

mentioned in merge request !98815 (merged)

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closed

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