Remove the concept of core builders from webkitpy

Remove the concept of core builders from webkitpy
https://bugs.webkit.org/show_bug.cgi?id=75809

Reviewed by Ryosuke Niwa.

Treat all bots equally since core/non-core sepration has been removed from build.webkit.org,
and remove the relevant code.

* Scripts/webkitpy/common/net/buildbot/buildbot.py:
(BuildBot.__init__):
(BuildBot._matches_regexps):
(BuildBot._builder_statuses_with_names_matching_regexps):
(BuildBot.builder_statuses):
(BuildBot.failure_map):
(BuildBot._latest_builds_from_builders):
(BuildBot.last_green_revision):
* Scripts/webkitpy/common/net/buildbot/buildbot_mock.py:
(MockBuildBot.builder_statuses):
* Scripts/webkitpy/common/net/buildbot/buildbot_unittest.py:
(test_status_parsing):
(test_last_green_revision):
(test_last_green_revision.mock_builds_from_builders):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@104417 268f45cc-cd09-0410-ab3c-d52691b4dbfc