Disable AnchorCheck plugin

Can't be relied on. Multiple reports of expected results not returned.

https://github.com/linkchecker/linkchecker/issues/542
https://github.com/linkchecker/linkchecker/issues/555
https://github.com/linkchecker/linkchecker/issues/568

Previously a fix was needed just to get the tests working:
0912e8a2c ("Don't strip the URL fragment from cache key if using AnchorCheck", 2020-07-27)

After:
eaa538c81 ("don't check one url multiple times", 2016-11-09)

linkcheck/plugins/anchorcheck.py

@@ -26,8 +26,16 @@ from ..htmlutil import linkparse
 class AnchorCheck(_ContentPlugin):
     """Checks validity of HTML anchors."""
 
+    def __init__(self, config):
+        """Initialize plugin."""
+        super().__init__(config)
+        log.warn(
+            LOG_PLUGIN, _("Anchor check plugin is broken. Fixes welcome.")
+        )
+
     def applies_to(self, url_data):
         """Check for HTML anchor existence."""
+        return False  # XXX Plugin disabled
         return url_data.is_html() and url_data.anchor
 
     def check(self, url_data):

tests/checker/test_anchor.py

@@ -16,6 +16,8 @@
 """
 Test html anchor parsing and checking.
 """
+import pytest
+
 from . import LinkCheckTest
 from .httpserver import HttpServerTest
 
@@ -25,6 +27,7 @@ class TestAnchor(LinkCheckTest):
     Test anchor checking of HTML pages.
     """
 
+    @pytest.mark.xfail(strict=True)
     def test_anchor(self):
         confargs = {"enabledplugins": ["AnchorCheck"]}
         url = "file://%(curdir)s/%(datadir)s/anchor.html" % self.get_attrs()
@@ -46,6 +49,7 @@ class TestHttpAnchor(HttpServerTest):
     Test checking of HTML pages containing links to anchors served over http.
     """
 
+    @pytest.mark.xfail(strict=True)
     def test_anchor_html(self):
         confargs = dict(enabledplugins=["AnchorCheck"], recursionlevel=1)
         self.file_test("http_anchor.html", confargs=confargs)