Fix unintentional selection mode entering by clicking
This one-liner removes the ability to enter selection mode by click-and-holding with a pointing device like a mouse.
This functionality was originally implemented because it seemed useful to save users the effort of entering selection mode explicitly by using its corresponding action.
However, click-and-holding to trigger anything is not really an expected behaviour. (This contrasts with touch devices where press-and-holding is common to trigger something.)
Aside from the above reasoning, the click-and-hold behaviour was also buggy so that selection mode was entered in a couple of situations that weren't strictly about click-and-holding.
So this commit removes the functionality and the bugs.