Don't print extra parens around expressions in extended stats

Don't print extra parens around expressions in extended stats

The code printing expressions for extended statistics doubled the
parens, producing results like ((a+1)), which is unnecessary and not
consistent with how we print expressions elsewhere.

Fixed by tweaking the code to produce just a single set of parens.

Reported by Mark Dilger, fix by me. Backpatch to 14, where support for
extended statistics on expressions was added.

Reported-by: Mark Dilger
Discussion: https://postgr.es/m/20210122040101.GF27167%40telsasoft.com