Remove guards for tf-psa-crypto/include/mbedtls conditionals

[felixc-arm]

Now that Mbed-TLS/TF-PSA-Crypto#247 is merged, the conditionals added in #10122 to account for both scenarios (i.e. when tf-psa-crypto/include/mbedtls exists with files and when it doesn't exist) can be removed.

n.b. Once this is merged, patches in TF-PSA-Crypto will need to be rebased on top of Mbed-TLS/TF-PSA-Crypto#247 to pass the CI. Perhaps it makes sense to wait a bit before merging this patch as it's not an important change and will save extra rebases for PRs that are not based on top of Mbed-TLS/TF-PSA-Crypto#247.
^^ This is probably outdated as it has been long enough that active patches likely would have been rebased for other reasons.

PR checklist

• changelog not required because: minor internal change
• development PR provided: HERE
• TF-PSA-Crypto PR not required because: mbedtls minor change only
• framework PR not required
• 3.6 PR not required because: 4.0/1.0 work
• tests not required because: minor non-functional change