Drop use of logs_severity needle from Cockpit test entirely

This match keeps giving us problems, and now I look at the test
again, there is actually no need for it at all. Clicking it
doesn't do anything, and we already confirm that we're on the
right page at the next step, where we look for a log entry and
click on that - that will fail if we aren't actually on the
Logs page.

I don't remember what Cockpit used to look like when we first
put this line and needle in, presumably there's a reason we had
them, but they're clearly unnecessary now.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

needles/cockpit/logs_severity-20191129.json

@@ -1,16 +0,0 @@
-{
-  "area": [
-    {
-      "type": "match",
-      "ypos": 160,
-      "width": 62,
-      "height": 16,
-      "xpos": 466,
-      "match": 94
-    }
-  ],
-  "properties": [],
-  "tags": [
-    "cockpit_logs_severity"
-  ]
-}

needles/cockpit/logs_severity-20191206.json

@@ -1,16 +0,0 @@
-{
-  "area": [
-    {
-      "height": 16,
-      "width": 62,
-      "xpos": 556,
-      "ypos": 160,
-      "type": "match",
-      "match": 92
-    }
-  ],
-  "properties": [],
-  "tags": [
-    "cockpit_logs_severity"
-  ]
-}

tests/server_cockpit_basic.pm

@@ -13,8 +13,6 @@ sub run {
     assert_and_click "cockpit_logs";
     # the date dropdown changes and messes with the button locations, so wait
     wait_still_screen 2;
-    assert_and_click "cockpit_logs_severity";
-    wait_still_screen 2;
     # assume there's an entry, click it
     assert_and_click "cockpit_logs_entry";
     # check we get to the appropriate detail screen