CronInterval: Remove unnecessary escapes in the "is this a calculation" regex.
CronInterval: Remove unnecessary escapes in the "is this a calculation" regex.

Except for a rare few, special characters in a regex character class `[]` do not need to be escaped.

mattyrob/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,mattyrob/WordPress-Coding-Standards,mattyrob/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,mattyrob/WordPress-Coding-Standards,mattyrob/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards