Remove doubly defined symbol from mbed_lib.json

[rwalton-arm]

Summary of changes

MBEDTLS_CMAC_C already seems to be defined in https://github.com/ARMmbed/mbed-os/blob/master/features/mbedtls/inc/mbedtls/config.h#L2416 - this was causing warnings in CMake builds about multiple defined symbols.

This patch removes the macro from the mbed_lib.json as it seems to be unnecessary.

Impact of changes

Migration actions required

Documentation

None

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

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

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Reviewers

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[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 6 of 6 test jobs passed
Build number : 1
Build artifacts