Remove EKT

[bifurcation]

The EKT code in libsrtp has long been incorrect and unusable. This PR removes it from the code base.

Fixes #384

[paulej]

Who would be relying on this public API given it doesn't function? It seems more confusing keeping it than it, especially since there is a new RFC for EKT.

[pabuhler]

@paulej hopefully no one and removing the header file to force that fact that it is broken is a good thing. Removing the header may result in the need for some application to recompile with he new version if they where including it for some reason and that is fine. The problem I think is the removal of "srtp_ekt_policy_t ekt;" this will change the size and offsets of the srtp_policy_t struct which would be a problem if an application uses a prebuilt distribution of libsrtp, such a change should be done in a major version only.

[bifurcation]

@pabuhler The point about changing the ABI is a good one. I've updated the PR so that there's a deprecated_ekt field in the srtp_policy_t struct where the ekt field was before. In order to keep the file deleted, I changed the type of the field to void*, but pointers are pointers, so it should maintain the layout of the rest of the struct.

[pabuhler]

@bifurcation there are two failed builds, one is clang format and that should be fixed, the other can probably be ignored for now.

[bifurcation]

@pabuhler Looks like all the builds are passing now except for the one where the tooling is broken.