Remove DH keygen special case

BC added an optimization for a BC-specific key generation params class
that ultimately causes Wycheproof's DhTest to fail.  While I believe
it would be safe to include that optimization, in the interest of
being conservative, I'm going to disable it to maintain the behavior of
previous versions.

Bug: 124926190
Test: cts -m CtsLibcoreTestCases
Test: cts -m CtsLibcoreWycheproofBCTestCases
Change-Id: I6aa9945d1e0f55cc4789d35edd41cc47fe61a3d6
2 files changed