Remove unused s2n_config_client_hello_cb_enable_poll

[lrstewart]

Description of changes:

s2n_config_client_hello_cb_enable_poll was initially added to make the ClientHello callback follow the polling model that Rust uses. However, that solution wouldn't have scaled to other async callbacks, and we went with a different solution. s2n_config_client_hello_cb_enable_poll complicates the already complex ClientHello callback logic, so I'd like to remove it.

Here's the commit that added it: 70884dc#diff-4a6a5adbf833174fde04f3043fd5575868bebc5085b7dee3c71abb4270c8d900

It's old enough that I couldn't git revert it without an unreasonable number of conflicts, so I manually reverted it.

Testing:

Existing tests pass. I removed tests that turned on the polling feature, but kept the extra HRR tests.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[harrisonkaiser]

For the future can you comment with a link to what our alternate solution is?

[lrstewart]

For the future can you comment with a link to what our alternate solution is?

This comment explains it, but the actual implementation is spread over several different commits.