Remove suspicious group inside type parameter character class

This group was added in 06c314bdb1d876e6dd4d3df64458c4462e6d575e. I'm not sure what the intent was here. Oniguruma parses `(?:+)` as regular characters. `[,]` is parsed as a nested character class and so is equivalent to just a bare comma. So the addition of this group to the character class really just means that parentheses and colons are now part of the (negated) class. I don't believe this was the intent, so I've removed the group entirely.