remove fullscreen option in right click menu

In a slideshow it makes sense to have a fullscreen option, but the slideshow will be a separated app in the future.
So having this option in pantheon-photos won't be necessary anymore.

Launchpad Details: [#LP1406555](https://bugs.launchpad.net/bugs/1406555) swizzle - 2014-12-30 14:50:25 +0000

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No file or test is named. Locate the right-click menu implementation for pantheon-photos, confirm where the fullscreen option is added, and check how the slideshow menu differs. Done means the fullscreen option is absent from the photo app's right-click menu while slideshow behavior remains accounted for.

Written by the indexing model from the issue text.