Remove unused config option

[MorrisJobke]

Found while looking through the code.

[codecov]

Codecov Report

Merging #9276 into master will decrease coverage by 2.06%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #9276 +/- ##
============================================
- Coverage 54.01% 51.94% -2.07% - Complexity 24075 25375 +1300 
============================================
Files 1513 1607 +94 Lines 88690 95394 +6704 Branches 1394 1394 ============================================
+ Hits 47905 49557 +1652 - Misses 40785 45837 +5052

Impacted Files	Coverage Δ	Complexity Δ
config/config.sample.php	0% <ø> (ø)	0 <0> (ø)	⬇️
.../tests/Unit/Collaboration/CommentersSorterTest.php	25.55% <0%> (-66.45%)	6% <0%> (ø)
apps/sharebymail/tests/SettingsTest.php	52.17% <0%> (-47.83%)	3% <0%> (ø)
lib/private/Security/RateLimiting/Limiter.php	55.55% <0%> (-44.45%)	5% <0%> (ø)
settings/Controller/GroupsController.php	68.33% <0%> (-31.67%)	9% <0%> (ø)
...b/private/App/AppStore/Fetcher/CategoryFetcher.php	69.23% <0%> (-30.77%)	1% <0%> (ø)
...ps/comments/tests/Unit/AppInfo/ApplicationTest.php	69.56% <0%> (-30.44%)	4% <0%> (ø)
apps/user_ldap/lib/Configuration.php	43.68% <0%> (-29.21%)	87% <0%> (ø)
lib/private/AvatarManager.php	71.42% <0%> (-28.58%)	4% <0%> (ø)
apps/encryption/lib/Command/EnableMasterKey.php	75% <0%> (-25%)	5% <0%> (ø)
... and 278 more