doc: clean up deprecations.md doc.
This removes the --pending-deprecation label from the Buffer() constructor in the deprecation.md docs.
It was added originally, since it was just a Documentation only deprecation. It is now a full runtime deprecation, which by default will output what the flag does anyway, so it can probably be removed
While i was starting to add docs for https://github.com/nodejs/node/pull/29671, i noticed this.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines