Revert changes to underscore's isEqual

[handeyeco]

Summary:

Original comment

I made a separate PR because I made this mistake in a couple of PRs so I thought I'd knock them out all at once.

Issue: LEMS-2737

Test plan:

[github-actions]

npm Snapshot: Published

Good news!! We've packaged up the latest commit from this PR (3bf118d) and published it to npm. You
can install it using the tag PR2125.

Example:

yarn add @khanacademy/perseus@PR2125

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

./dev/tools/bump_perseus_version.sh -t PR2125

[github-actions]

Size Change: +2 B (0%)

Total Size: 1.47 MB

Filename	Size	Change
packages/perseus-core/dist/es/index.js	27.1 kB	+1 B (0%)
packages/perseus-score/dist/es/index.js	113 kB	+1 B (0%)

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	86.8 kB
packages/math-input/dist/es/index.js	77.6 kB
packages/math-input/dist/es/strings.js	1.79 kB
packages/perseus-editor/dist/es/index.js	689 kB
packages/perseus-linter/dist/es/index.js	22.2 kB
packages/perseus/dist/es/index.js	394 kB
packages/perseus/dist/es/strings.js	4.87 kB
packages/pure-markdown/dist/es/index.js	3.66 kB
packages/simple-markdown/dist/es/index.js	12.5 kB

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