[3.13] DOCS: fix error in exec namespace note (gh-119380)

When updating the new exec note added in gh-119235 as part of the
PEP 667 general docs PR, I suggested a workaround that isn't valid.

The first half of the note is still reasonable, so just omit the invalid text.

(cherry picked from commit 31d61a75c9ae8c1b1bc6447f517122be8adce2ef)

Co-authored-by: Alyssa Coghlan <ncoghlan@gmail.com>