[kcms/lookandfeel] Remove unused Customize button
requested to merge ngraham/plasma-workspace:remove-unused-customize-button-in-globalthemes-kcm into master
This button is left over from an earlier version of the new design and is redundant with the sidebar list of child KCMs. It also doesn't work properly (the menu doesn't stay open after the first click and the menu items don't do anything)