Update changelog

Author: aeisenberg

extensions/ql-vscode/CHANGELOG.md

@@ -9,6 +9,7 @@
 - Remember previous sort order when reloading query results.
 - Fix proper escaping of backslashes in SARIF message strings.
 - Allow setting `codeQL.runningQueries.numberOfThreads` and `codeQL.runningTests.numberOfThreads` to 0, (which is interpreted as 'use one thread per core on the machine').
+- Clear the problems view of all Code QL problems when a database is removed.
 
 ## 1.3.3 - 16 September 2020