Remove MBEDTLS_xxx_ALT interface for crypto mechanisms

[tom-daubney-arm]

Description

This PR aims to remove the support for the _ALT interface for cryptographic mechanisms.

PR checklist

Please tick as appropriate and edit the reasons (e.g.: "backport: not needed because this is a new feature")

• [ ] changelog TODO
• [x] 3.6 backport not required
• [x] 2.28 backport not required
• [x] tests existing tests suffice

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