Remove rule 6.D.7 entirely

The rule is confusing to many players, and the rule requires that players in some situations call balls as "out" that they know for a fact are "in".

Also: there is a very subtle distinction between "the ball is in" and "I as an observer cannot call the ball out."  This subtlety is lost on many players, and in my observation is that the rule is commonly misunderstood, thus is a problematic rule.  Many players claim that a ball is "in" unless a space is visible.  Many players construe this rule to mean that line calls are to be made as they are in soccer: if any part of the ball is hovering over the line, the ball is in.  The rule has also caused confusion since a player with one vantage point will claim that the ball is "in" since they can't see a gap, whereas another player who can see a gap will claim that the ball is "out".  The fact of the matter is that the ball is either in or it's out; whether an observer is positioned correctly does not change the state of the ball.

My overall observation is that the rule has failed at making line calls better, and has on the contrary made line calls worse.  The rulebook and the game are better off without this rule.