Remove Meta+Tab as default activity shortcut

It seems to me that activities are a rarely used feature of plasma and most users do not fully understand how to use them; although I'd like to address this situation (I'm a big fan of activities myself) as I've explained in the kde-devel mailing list, I think it doesn't make much sense to have such an important shortcut (Meta+Tab) on a rarely used feature. On Windows it brings up the window switcher and on macOS command+tab is used to switch through applications. This patch, together with (kwin!478 (closed)) make Plasma more consistent with other OSs and makes two very useful features (desktops grid, which also works as a window switcher, and present windows) more evident to the user.