Remove unnecessary cl-case quotes

[dsedivec]

Emacs master is now warning when you incorrectly include quotes in cl-case forms. Loading ace-window currently yields:

ace-window.el: Warning: Case 'visible will match ‘quote’.  If that’s intended, write (visible quote) instead.  Otherwise, don’t quote ‘visible’.
ace-window.el: Warning: Case 'global will match ‘quote’.  If that’s intended, write (global quote) instead.  Otherwise, don’t quote ‘global’.
ace-window.el: Warning: Case 'frame will match ‘quote’.  If that’s intended, write (frame quote) instead.  Otherwise, don’t quote ‘frame’.

I believe this PR will fix those errors.

Thank you for all your software, including ace-window. :)

[rennsax]

+1 for this