n-api: fix compiler warning

[cjihrig]

private field 'async_context' is not used [-Wunused-private-field]

Refs: #17887

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

[nodejs-github-bot]

@cjihrig build started: https://ci.nodejs.org/blue/organizations/jenkins/node-test-pull-request-lite-pipeline/detail/node-test-pull-request-lite-pipeline/179/pipeline

[addaleax]

LGTM; but: @gabrielschulhof I thought you added async context tracking? I’m not sure where that’s being propagated here…

[gabrielschulhof]

@addaleax I believe I simply forgot to remove that field when class TsFn started inheriting from AsyncResource and therefore did no longer need to store its async_context.

[danbev]

CI: https://ci.nodejs.org/job/node-test-pull-request/15703/

[danbev]

node-test-commit-windows-fanned failure looks unrelated

console output:

09:30:47 not ok 566 sequential/test-inspector-break-when-eval
09:30:47   ---
09:30:47   duration_ms: 0.403
09:30:47   severity: fail
09:30:47   exitcode: 1
09:30:47   stack: |-
09:30:47     [test] Connecting to a child Node process
09:30:47     [test] Testing /json/list
09:30:47     [err] Debugger listening on ws://127.0.0.1:61103/56a876e9-c99e-4083-961c-f8ff452a4e8d
09:30:47     [err] For help, see: https://nodejs.org/en/docs/inspector
09:30:47     [err] 
09:30:47     [out] Ready!
09:30:47     [out] 
09:30:47     [test] Setting up a debugger
09:30:47     [test] Breaking in the code
09:30:47     [test] Step over console statement and test output
09:30:47     [out] 0 3
09:30:47     [out] 
09:30:47     [test] Verify node waits for the frontend to disconnect
09:30:47     [err] Debugger attached.
09:30:47     [err] Waiting for the debugger to disconnect...
09:30:47     [err] 
09:30:47     { AssertionError [ERR_ASSERTION]: Input A expected to strictly equal input B:
09:30:47     + expected - actual
09:30:47     
09:30:47     - 0
09:30:47     + 3221225477
09:30:47         at runTests (c:\workspace\node-test-binary-windows\test\sequential\test-inspector-break-when-eval.js:65:10)
09:30:47         at process._tickCallback (internal/process/next_tick.js:68:7)
09:30:47       generatedMessage: true,
09:30:47       name: 'AssertionError [ERR_ASSERTION]',
09:30:47       code: 'ERR_ASSERTION',
09:30:47       actual: 0,
09:30:47       expected: 3221225477,
09:30:47       operator: 'strictEqual' }
09:30:47     1
09:30:47   ...