[debugger] unskip mjsunit/debug-stepin-builtin-callback-opt.

This test does not seem to fail any longer.

R=machenbach@chromium.org
BUG=v8:4690
LOG=N

Review URL: https://codereview.chromium.org/1927543002

Cr-Commit-Position: refs/heads/master@{#35814}

test/mjsunit/mjsunit.status

@@ -751,9 +751,6 @@
 
 ##############################################################################
 ['ignition or ignition_turbofan', {
-  # TODO(yangguo,4690): flaky failures on the bots.
-  'debug-stepin-builtin-callback-opt': [SKIP],
-
    # TODO(rmcilroy,4765): assertion failures in LiveEdit tests.
   'debug-liveedit-restart-frame': [FAIL],
   'debug-liveedit-literals': [FAIL],