Drop unneeded OkHttp compile-time dependency on conscrypt

 - This dependency exists only in Android's version of OkHttp,
   rather than upstream.

 - It was added several years ago and doesn't seem to have been
   used lately. It shouldn't exist because the crypto library
   should be pluggable. Of course, a runtime dependency on
   conscrypt or a suitable replacement still exists.

 - Tests are still allowed to depend on conscrypt at compile time,
   although it should be kept to a minimum. At the moment, only
   PlatformTest.java has a compile time dependency on conscrypt;
   it imports com.android.org.conscrypt.OpenSSLSocketImpl.

Test: Nothing. Relying on treehugger to check that everything builds.

Bug: 29796778
Change-Id: I9570fd67cec6309fce5e0653a2d235ce6dc05b15