[rexx] Address simplicification comments in PR 2746

[RossPatterson]

@KvanTTT had a suggestion for simplifying the change I made in PR #2746. Since that PR was already merged, here's a new one. He already reviewed the change over in that PR. Basically, there was no need to introduce a new channel for the purpose of recognizing significant whitespace, so I reverted that part of the PR and checked the token type instead of the token's channel.