Fix crash with CJK keyboard with emoji at end of text field (#42539)

[Jasguerrero]

The isRTLAtPosition method had a bug, it used NSInteger max = [_selectionRects count] instead of NSInteger max = [_selectionRects count] - 1. But I realized we don't even need the function any more, it was used in a few places in previous iterations of #36643, but in the only place remaining, we actually already have the selection rect and don't need to search for it by position.

Btw as an explanation of the crash, I guess there is some mismatch between code point and character count somewhere. UIKit was asking for caretRectForPosition:2 when we only had 1 character. This could have only crashed when floating cursor selection was used, but actually when switching to CJK keyboard, UIKit turns out to use caretRectForPosition to calculate something about the composing rect.

Fixes flutter/flutter#128031

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