Remove SpdyPriorityForest, which is unused and obsolete.

This CL lands server change 95010541 by mpw.

BUG=488484

Review URL: https://codereview.chromium.org/1172103002

Cr-Commit-Position: refs/heads/master@{#333724}

net/net.gypi

@@ -1097,7 +1097,6 @@
       'spdy/spdy_pinnable_buffer_piece.h',
       'spdy/spdy_prefixed_buffer_reader.cc',
       'spdy/spdy_prefixed_buffer_reader.h',
-      'spdy/spdy_priority_forest.h',
       'spdy/spdy_priority_tree.h',
       'spdy/spdy_protocol.cc',
       'spdy/spdy_protocol.h',
@@ -1679,7 +1678,6 @@
       'spdy/spdy_network_transaction_unittest.cc',
       'spdy/spdy_pinnable_buffer_piece_test.cc',
       'spdy/spdy_prefixed_buffer_reader_test.cc',
-      'spdy/spdy_priority_forest_test.cc',
       'spdy/spdy_priority_tree_test.cc',
       'spdy/spdy_protocol_test.cc',
       'spdy/spdy_proxy_client_socket_unittest.cc',