Completely replace legacy pkey impls with EVP impl

[lrstewart]

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Problem:

Last remaining item from #5105

I replaced the sign/verify methods for all pkeys with the EVP sign/verify. However, we still use legacy logic for other options, like RSA encryption/decryption and the key match check.

Solution:

Rip out the rest of the old legacy pkey logic and replace it with EVP logic. This will be a nice simplification.

• Does this change what S2N sends over the wire? no
• Does this change any public APIs? no
• Which versions of TLS will this impact?

Requirements / Acceptance Criteria:

What must a solution address in order to solve the problem? How do we know the solution is complete?

• RFC links: Links to relevant RFC(s)
• Related Issues: Link any relevant issues
• Will the Usage Guide or other documentation need to be updated?
• Testing: How will this change be tested? Call out new integration tests, functional tests, or particularly interesting/important unit tests.
  • Will this change trigger SAW changes? Changes to the state machine, the s2n_handshake_io code that controls state transitions, the DRBG, or the corking/uncorking logic could trigger SAW failures.
  • Should this change be fuzz tested? Will it handle untrusted input? Create a separate issue to track the fuzzing work.

Out of scope:

Is there anything the solution will intentionally NOT address?