Rename dev_compiler/LICENCE.md -> LICENSE, remove obsolete text.

Change-Id: I18bed36eb433a5a053c026bb327a6a474fd4161d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153614
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Reviewed-by: Mark Zhou <markzipan@google.com>
Commit-Queue: David Morgan <davidmorgan@google.com>

pkg/dev_compiler/LICENSE.md → pkg/dev_compiler/LICENSE

@@ -1,11 +1,3 @@
-This license applies to all files contained here with the following
-exceptions:
-
-- All code in test/js_test_tools is externally maintained with
-  corresponding licenses in those individual files and directories.
-
----
-
 Copyright 2015, the Dart project authors. All rights reserved.
 
 Redistribution and use in source and binary forms, with or without