Removing unused permission SUBSITE_ASSETS_CREATE_SUBSITE

This isn't used, according to the description it would limit the list
of subsites you can choose to apply a File/Folder to. However, this
dropdown is shown to the user based on whether they have access to
that subsite, so this unused permission code isn't needed.
Merge pull request #154 from halkyon/irrelevant_permission_removal

Removing unused permission SUBSITE_ASSETS_CREATE_SUBSITE