[stable10] Remove not longer valid update script - pre-8.1 upgrade

[phil-davis]

Backport #33209

I noticed this when backporting "Introduce PHPstan" #35774 - in that PR I had to exclude this file from analysis because:

php -d zend.enable_gc=0 vendor-bin/phpstan/vendor/bin/phpstan analyse --memory-limit=2G --configuration=./phpstan.neon --level=0 apps core settings lib/private lib/public ocs ocs-provider
 1392/1392 

 ------ -------------------------------------------------------------------------------------------------------------------------------- 
  Line   apps/files/appinfo/update.php                                                                                                   
 ------ -------------------------------------------------------------------------------------------------------------------------------- 
  52     Function owncloud_reset_encrypted_flag not found while trying to analyse it - autoloading is probably not configured properly.  
  72     Function owncloud_reset_encrypted_flag not found.                                                                               
  79     Function owncloud_reset_encrypted_flag not found.                                                                               
 ------ -------------------------------------------------------------------------------------------------------------------------------- 

It is "ordinary" PHP, not a class.

Rather than trying to sort all that out, delete it.
Deleted code is debugged code.

[phil-davis]

@DeepDiver1975 does it seem OK to delete this for 10.3 ?
(rather than in a major version 11)