Drop the disable images feature

This removes the option from the Special:MobileOptions page and
all usages.

The feature is barely used and since the introduction of lazy
loading images is not worth the maintenance cost. Users who want
to disable images are advised to explore browser extensions/
use their browser.

Note this does not impact the mobileview API. Images can still
be scrubbed via noimages tag. MobileFormatter::doRemoveImages
has been retained (but can be removed in a later patchset if
necessary)

Changes:

• Remove public methods setDisableImagesCookie and imagesDisabled

from MobileContext

• The hook SpecialMobileEditWatchlist::images will now always run
• wgImagesDisabled config variable is no longer available

Additional changes:

• Drop some outdated FIXMEs for T87871 which has since been fixed
• Drop unused Browser::supportsFileUploads

Bug: T109870
Depends-On: I88fc77d6a443dc1713ef50f20a21cfbe29d532e4
Change-Id: Ia735cda4711eaacfe64ca27193fd9d5fd2b9910c