C#: Remove filter and external queries

These are legacy queries that are no longer used.

Author: hvitved

config/identical-files.json

@@ -376,7 +376,6 @@
   ],
   "DuplicationProblems.inc.qhelp": [
     "cpp/ql/src/Metrics/Files/DuplicationProblems.inc.qhelp",
-    "csharp/ql/src/Metrics/Files/DuplicationProblems.inc.qhelp",
     "javascript/ql/src/Metrics/DuplicationProblems.inc.qhelp",
     "python/ql/src/Metrics/DuplicationProblems.inc.qhelp"
   ],
@@ -435,4 +434,4 @@
     "javascript/ql/src/semmle/javascript/security/CryptoAlgorithms.qll",
     "python/ql/src/semmle/crypto/Crypto.qll"
   ]
-}
+}