Attempt at fixing copyright string in mockingbird prelude from trigge…

…ring OSS detectors

Summary:
certainly not the highest priority issue, but tackling this as part of ramping up and because it seemed easy

## context

It seems like [this line of code](https://github.com/facebook/buck2/blob/f49362c48418b31c3b603d6850e9bdf2762abb7f/prelude/apple/mockingbird/mockingbird_mock.bzl#L49) is triggering warnings when folks try to use buck2 in their company ([reported in the discord here](https://discord.com/channels/1100487223746510850/1100487225889796118/1327268605364015145))

{F1974381444}

## this diff

[string concatenation](https://bazel.build/rules/lib/core/string) seems pretty straightforward in bazel so I'm trying a pretty simple (naive?) way of "solving" this issue in the same way that [we've told people for years](https://fb.workplace.com/groups/phabricator/posts/5826257154051995) to "solve" the issue of their code being seen as generated because they have `generated` somewhere in the file

Reviewed By: chatura-atapattu

Differential Revision: D68241905

fbshipit-source-id: e2d5cdd27f3e5849d6e75c0026e3f20a06beb895

prelude/apple/mockingbird/mockingbird_mock.bzl

@@ -45,8 +45,6 @@ def _impl(ctx: AnalysisContext) -> list[Provider]:
         json_project_description_output,
         "--output",
         mockingbird_source.as_output(),
-        "--header",
-        "// (c) Meta Platforms, Inc. and affiliates. Confidential and proprietary.",
         "--support",
         ctx.attrs._mockingbird_support[DefaultInfo].default_outputs,
         "--verbose",