mail/squirrelmail-sasql-plugin: Remove extra escapes

This port currently uses some extra escapes in its sed regex: \$$
and \'. The latter is considered an escape of an ordinary character,
and will be disallowed in a future version of regex(3) while we
segue into libregex(3) repurposing some of these escapes for special
meaning.

PR: 233445
Submitted by: kevans