Repair wp/6.4 branch Actions: remove the mobile-unit-js job - #409

The `mobile-unit-js` job (displayed as `Mobile`) runs
`npm run test:native`, the Jest suite for the React Native mobile app.
It is not disabled on this branch and it is not broken: it carries the
ordinary `if: ${{ github.repository == 'WordPress/gutenberg' ||
github.event_name == 'pull_request' }}` condition, runs on every pull
request against this stack, and currently passes in about five and a
half minutes.

It is removed for scope and consistency, not because it fails. #109
already removed `rnmobile-ios-runner.yml`, `rnmobile-android-runner.yml`
and `gradle-wrapper-validation.yml` from this branch on the grounds that
React Native mobile CI does not belong on a frozen release branch: the
mobile app is released from its own repository on its own schedule and
is never built from `wp/6.4`, so nothing here can regress it and no
result produced here is ever consulted. `mobile-unit-js` is the last
remaining piece of that same mobile CI surface, and leaving it behind
means this branch keeps paying for a runner on every pull request to
test a target it does not ship.

No other job references it. Nothing in the workflow uses `needs:
[mobile-unit-js]`, and no job's `outputs:` existed only to feed it, so
the removal is self-contained and the remaining four jobs (`unit-js`,
`test-php`, `phpcs`, `unit-php`) are untouched.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DtSAAJkQ9P6vPtDumqStg8

# Conflicts:
#	.github/workflows/unit-test.yml