ICU-21179 Clean up unused code in the RBBI dictbe

ICU-21179 Clean up unused code in the RBBI dictbe

The wordsMatched conditional gave the misleading impression that only the first "first word" candidate that was followed by a dictionary word would be marked, but because the wordsMatched declaration is inside the while loop, every "first word" candidate that's followed by a dictionary word is marked (resulting in the shortest and not the longest first candidate that's followed by a second dictionary word).