Update adobe-photoshop-lightroom: remove uninstall_preflight

[commitay]

This is also break when Homebrew/brew#3396 is merged.

As this is a patch installer on top of another Cask it doesn't need an uninstall itself, the app can be removed by uninstalling lightroom600.

The patch upgrade can be applied on top of the previous patch.

[commitay]

>>> brew cask _audit_modified_casks 5c0d832f7d5846170347cc6c573400160379ba5c...a4b7d4a84cd0833500349fcce205256e7f4857ff
==> 1 modified cask: adobe-photoshop-lightroom
audit for adobe-photoshop-lightroom: passed

>>> brew cask style Casks/adobe-photoshop-lightroom.rb
1 file inspected, no offenses detected

[commitay]

Going to leave this as is for now. It won't actually "break" the upgrade, it will just install a clean version and then patch it.