validate: remove known issue for mxfdemux that's fixed now

https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/issues/1020

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4659>

subprojects/gst-integration-testsuites/testsuites/validate_known_issues.py

@@ -229,19 +229,6 @@ KNOWN_ISSUES = {
             },
         ],
     },
-    "https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/issues/1020": {
-        "tests": [
-            "validate.*mxf.*",
-        ],
-        "issues": [
-            {
-                "issue-id": "threading::pull-range-from-wrong-thread",
-                "summary": "gst_pad_pull_range called from wrong thread",
-                "level": "critical",
-                'can-happen-several-times': True,
-            },
-        ]
-    },
     "Our asf file is basically broken": {
         "tests": [
             "validate.file.transcode.*.samples_multimedia_cx_asf_wmv_elephant_asf",