This commit 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.

BUG: 457973   
BUG: 458129   
CCBUG: 457975