Simplify record reading in ssl_parse_client_hello()

[mpg]

As a follow-up to #4156 which removes support for parsing SSLv2 ClientHello messages, we can now finally get rid of custom record reading in ssl_parse_client_hello() and instead use mbedtls_ssl_read_record() like all other ssl_parse_xxx() functions.

Expected benefits: small code size gain, better maintainability, and more importantly, unblock MPS work.

[hanno-becker]

This should be done before #4823

[ronald-cron-arm]

I've moved it to the 3.x as I believe this is not related or necessary for the TLS 1.3 implementation.

[mpg]

I'm not sure if where we track MPS work, but perhaps this could be put there as well? I don't know all the details, but I'd be surprised if this wasn't a pre-requisite for moving to MPS, as this is about removing a layering violation.

[mpg]

Note: what's the impact in terms of allocating state when we receive a fragmented ClientHello? What do we want to happen in this case? See second point here: #7549 (comment)