Fix URLConnectionTest for devices without full internet access.

This removes two bad tests that are covered elsewhere, using
mocks.

test_getContentEncoding() was hitting http://www.amazon.com, to see
if a URLConnection getter was working. That getter is now tested in
libcore/luni/src/test/java/libcore/java/net/URLConnectionTest.java,
in testClientConfiguredGzipContentEncoding().

test_getLastModified() was hitting
http://www.php.net/manual/en/function.explode.php, to see if
getHeaderFieldDate was working for Last-Modified. That getter is
better tested in test_getHeaderFieldDateLjava_lang_StringJ(), in the
same file.

Bug:18480776
Change-Id: I21ffc5c9c72eb4379478c11687d197dab176a851