Remove locked-figures-aria flag

[nishasy]

Summary:

Remove the locked-figures-aria flag from the Perseus repo
now that the locked figures' aria labels have been out for
a while now.

Issue: https://khanacademy.atlassian.net/browse/LEMS-2274

Test plan:

yarn jest

[github-actions]

npm Snapshot: Published

Good news!! We've packaged up the latest commit from this PR (d7ffeac) and published it to npm. You
can install it using the tag PR2064.

Example:

yarn add @khanacademy/perseus@PR2064

If you are working in Khan Academy's webapp, you can run:

./dev/tools/bump_perseus_version.sh -t PR2064

[github-actions]

Size Change: -132 B (-0.01%)

Total Size: 1.27 MB

Filename	Size	Change
packages/perseus-editor/dist/es/index.js	688 kB	-34 B (0%)
packages/perseus/dist/es/index.js	417 kB	-98 B (-0.02%)

ℹ️ View Unchanged

Filename	Size
packages/kas/dist/es/index.js	39 kB
packages/keypad-context/dist/es/index.js	760 B
packages/kmath/dist/es/index.js	4.27 kB
packages/math-input/dist/es/index.js	78 kB
packages/math-input/dist/es/strings.js	1.79 kB
packages/perseus-core/dist/es/index.js	1.48 kB
packages/perseus-linter/dist/es/index.js	22.2 kB
packages/perseus/dist/es/strings.js	4.93 kB
packages/pure-markdown/dist/es/index.js	3.67 kB
packages/simple-markdown/dist/es/index.js	12.5 kB

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