Improve collection tree keyboard navigation.
A comment from 2008 notes that "L and R should magically work when we get a patched version of qt". Well, a patched version has since arrived. This custom left and right key handling makes the collection treeview behave differently from normal treeviews, lacking the ability to close parent items when selected is not expanded, and focusing first child item when an right key is pressed on first expanded item.
The other keypress handlers still make sense, so just removing these two to improve functionality.
To test: Without patch: Select an item in collection tree view. Press right arrow: The item gets expanded. Press right arrow again: Nothing happens. With this MR: Select an item in collection tree view. Press right arrow: The item gets expanded. Press right arrow again: The first child is selected.
Without patch: Select an item in collection tree view. Press left arrow: The item gets collapsed. Press left arrow again: Nothing happens. With this MR: Select an item in collection tree view. Press left arrow: The item gets collapsed. Press left arrow again: The parent index is selected.
This makes Collection browser keyboard left+right navigation correspond to the normal Qt behaviour, which is also observable on e.g. Playlist and Podcast browsers.