[4.0] Remove show_associations field of association is not enabled

[joomdonation]

Pull Request for Issue # .

Summary of Changes

This simple PR removes the field show_associations field of association is not enabled. It causes fatal error while saving article when I tested PR #12414 and @Hackwar had to add a check (which I think not necessary) to prevent the error.

Plus, it also removes some unused variables/lines of code which can be seen easily if you use a good IDE like phpstorm.

Testing Instructions

Apply patch, try to save article and confirm nothing broken. Maybe code review should be enough.

[infograf768]

I have tested this item ✅ successfully on 802acf8

ok on review.

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[astridx]

I applied this patch and I could create and save an article.

I see no change in the options tab of com_content.
The field show_associations is shown when the Plugin System - Language Filter is active and it is not shown when the Plugin System - Language Filter is not active - with and without this patch.

Perhaps we can delete the line

joomla-cms/administrator/components/com_content/Model/ArticleModel.php

Line 905 in 88d5f95

$workflowId = 0;

in this PR, too.

[astridx]

I have tested this item ✅ successfully on 88d5f95

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[wilsonge]

@zero-24 RIPS please

[zero-24]

@zero-24 RIPS please

RIPS seams to be fine?

[wilsonge]

Thanks!