Remove `filterHttpHeaders` config option

[david-luna]

The agent is no longer using filterHttpHeaders config option to redact cookies. This pull request changed the data format to place the cookies inside context.request.cookies map which is accepted by the intake API.

With that change now sanitizeFieldNames options is in charge of redacting the cookies and there is no use for filterHttpHeaders which is already marked as deprecared.

We should then remove the config option

[trentm]

Notes: perhaps add "deprecated" note in index.d.ts for this option as well.

[trentm]

Closed by #3539 on "dev/4.x" branch, which will be the 4.x active branch soonish.