Remove jquery ui background image

[juliusknorr]

Remove leftover jquery ui background image, which was causing a request to a non-existing file:

GET http://localhost:8140/core/vendor/jquery-ui/themes/base/images/ui-bg_highlight-soft_75_cccccc_1x100.png 404 (Not Found)

Steps to reproduce:

1. Upload a file so that the jquery ui progressbar comes up
2. watch the browser console

@nextcloud/designers

[rullzer]

🙈

[codecov]

Codecov Report

Merging #8165 into master will decrease coverage by 16.65%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #8165 +/- ##
=============================================
- Coverage 51.73% 35.08% -16.66% 
Complexity 25357 25357 =============================================
Files 1598 1598 Lines 95021 95021 Branches 1376 1376 =============================================
- Hits 49160 33336 -15824 - Misses 45861 61685 +15824

Impacted Files	Coverage Δ	Complexity Δ
apps/files/lib/Activity/Settings/FileDeleted.php	0% <0%> (-100%)	8% <0%> (ø)
apps/user_ldap/lib/GroupPluginManager.php	0% <0%> (-100%)	17% <0%> (ø)
apps/files/lib/Activity/Settings/FileCreated.php	0% <0%> (-100%)	8% <0%> (ø)
lib/public/AppFramework/Http/DownloadResponse.php	0% <0%> (-100%)	1% <0%> (ø)
apps/files/lib/Activity/Settings/FileRestored.php	0% <0%> (-100%)	8% <0%> (ø)
lib/public/RichObjectStrings/Definitions.php	0% <0%> (-100%)	2% <0%> (ø)
apps/user_ldap/lib/UserPluginManager.php	0% <0%> (-100%)	24% <0%> (ø)
apps/files_trashbin/appinfo/app.php	0% <0%> (-100%)	0% <0%> (ø)
lib/private/App/AppStore/Version/Version.php	0% <0%> (-100%)	3% <0%> (ø)
apps/federation/lib/Hooks.php	0% <0%> (-100%)	4% <0%> (ø)
... and 516 more

[danxuliu]

Tested and works 👍; changes make sense to override the default background image from jQuery UI.

Failing tests are not related.

[juliusknorr]

stable13: #8287