Remove vestiges of "memories" in brave://flags

[stephendonner]

Description

Remove the last vestiges of "memories" in brave://flags

This is a followup to #15582

Steps to Reproduce

1. new profile
2. launch Brave
3. load brave://flags
4. type "memories"

Actual result:

Expected result:

Nothing listed/found

Reproduces how often:

100%

Brave version (brave://version info)

Brave	1.31.10 Chromium: 93.0.4577.58 (Official Build) nightly (x86_64)
Revision	c4410ece044414ea42fa4ba328d08195e818a99c-refs/branch-heads/4577@{#1076}
OS	macOS Version 11.5.2 (Build 20G95)

/cc @goodov

[rebron]

Closing as stale. Flags look like they've been removed.