Remove browserstack configuration

Editor assessment

The subject says exactly what was removed — BrowserStack configuration — and the body, while mostly a restatement, appends the essential reason: it was not used for the project's CI. That single clause converts a mechanical deletion into an understandable decision. Type `ci` is accurate; scope is absent but the subject's specificity compensates. The `PR Close #43881` footer is well formed and meaningful as the merge reference. Style hygiene is solid: imperative mood, no trailing period, no pasted logs or diff fragments, consistent language. The message teaches little beyond the removal itself, which caps its teaching value at good rather than exemplary.