remove docs for unimplemented character class combinators

[coke]

from https://rt.perl.org/Ticket/Display.html?id=130419 opened by @briandfoy

The |, ^, and & character class (and Unicode property) set operators
aren't parseable:

    > / <[abc] | [def]> /
    ===SORRY!=== Error while compiling:
    Unable to parse expression in metachar:sym<assert>; couldn't find final '>'
    ------> / <[abc]⏏ | [def]> /
    > / <[abc] ^ [def]> /
    ===SORRY!=== Error while compiling:
    Unable to parse expression in metachar:sym<assert>; couldn't find final '>'
    ------> / <[abc]⏏ ^ [def]> /
    > / <[abc] & [def]> /
    ===SORRY!=== Error while compiling:
    Unable to parse expression in metachar:sym<assert>; couldn't find final '>'
    ------> / <[abc]⏏ & [def]> /
    > / <[abc] + [def]> /
    / <[abc] + [def]> /
    > / <[abc] - [def]> /
    / <[abc] - [def]> /

These are documented in https://docs.perl6.org/language/regexes but are
not marked as "not implemented" or something similar.

And a followup by @jnthn:

I doubt they'll be implemented particularly soon; I suggest they are removed from the docs.