LWIP: Remove unused MBOX configs

[michalpasztamobica]

Summary of changes

Remove the two MBOX config options which were ignored by LWIP. The configs have never been used (sys_mbox_new() silently ignores them), so I doubt this is a breaking change and did not mark it as such.
Fixes #12046

Impact of changes

Some mbed_app.json files may not compile if they used the removed options.

Documentation

Not required

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[x] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@pstolarz
@kjbracey-arm
@AnttiKauppila

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[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts

[adbridge]

@michalpasztamobica I've updated the template for you to show how the description should be added. These fields will be automatically parsed in the future so it would be helpful if you could use the template as designed. Many thanks .