Simplify EXPLAIN code for Memoize

Simplify EXPLAIN code for Memoize

This removes a needless special case for Memoize's FORMAT TEXT EXPLAIN
output.

ExplainPropertyText() outputs the same thing in text mode as the
special-case code was doing, so removing the special-case code results in
the same EXPLAIN output, just with less code.

It seems like a good idea to fix this to help prevent future changes in
this area from copying the same pattern.

Author: Ilia Evdokimov <ilya.evdokimov@tantorlabs.com>
Reported-by: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/88a71bcd-0b5c-4d0b-8107-757e96f402d5@tantorlabs.com