timeline: Remove previous/next keyframe menu items

The logic is missing and we are not sure how useful they are.
The unittests when the functionality has been added are long gone:
efa26eb0
timeline: Remove obsolete FIXME

The bug is marked as NOTABUG and the code works fine.
timeline: Cleanup the logic which allocates a ghostclip to a layer
timeline: Fix dragging an asset to the ruler

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1061824