[MWF] Fix bug in DataGridViewRowCollection Clear() introduced by recent commit

7d1b878fbf13462a282860c6d1dd493a48d7b1eb altered the behavior of removing rows, breaking
the DataGridViewRowCollectionTest.ClearRows () test

Before the change, this is what happened when the list was cleared
(the number in brackets is the value of the Index property of the DataGridViewRow):

[0]  Remove   [1]  ReIndex  [0] Remove  [1] ReIndex  [0]
[1] --------> [2] --------> [1] ------> [2] -------> [1]  ...
[2]           [3]           [2]
[3]

After the offending commit, ReIndex is only called after the list is cleared, which means
the Index property of the rows is never updated (as there are no items in the list).

The fix is to explicitly set the Index of each row to 0.
This was the end result before the commit anyway, so we can omit the ReIndex call entirely.