Remove duplication of toggle-file-finder shortcut

[adius]

Why are there 2 shortcuts for toggle-file-finder?
It seems like it would make much more sense to use only cmd-T to toggle the file-finder and use cmd-P for something else. Maybe even replace cmd-shift-P with cmd-P, as it is "probably the most important command in Atom" (as stated in the flight manual) and should therefore better be assigned to a 2 key instead of a 3 key shortcut.

[winstliu]

Please see #78, #92, and #106, which all aim to solve this problem.