refactor(mbedtls): remove mbedtls 2.x support

Deprecation notice was sent to the community and no objection was
raised, so removing mbedtls 2.x support.

Change-Id: Id3eb98b55692df98aabe6a7c5a5ec910222c8abd
Signed-off-by: Lauren Wehrmeister <lauren.wehrmeister@arm.com>
5 files changed