Drop support for proxying FTP over HTTP

This feature accidentally launched in Android N; it relied on
9k lines of untested code in the packages
sun.net.www.{http,protocol.http} which are dropped by this CL.

Tests for FtpURLConnection that depend on external/mockftpserver
were added; the tests originally derive from draft CL
http://r.android.com/143327 but those have been refactored.
Additional tests were added to cover specifically FTP proxying
over HTTP, which is no longer supported after this CL.

Bug: https://code.google.com/p/android/issues/detail?id=160725
Bug: 29574477
Test: libcore.java.net.FtpURLConnectionTest
Test: cts-tradefed run cts -m CtsLibcoreTestCases -a arm64-v8a
Change-Id: I8a2aab36632e162e4616a4f3c61e833f63a97b88