[Gardening] Remove redundant --compiler=dartkb from test_matrix.json

Change-Id: I1bfc4fee0250cd087404f3593df812b7b7d96f79
Reviewed-on: https://dart-review.googlesource.com/c/86382
Reviewed-by: Martin Kustermann <kustermann@google.com>
Reviewed-by: William Hesse <whesse@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>
Commit-Queue: William Hesse <whesse@google.com>
Auto-Submit: Martin Kustermann <kustermann@google.com>

tools/bots/test_matrix.json

@@ -451,7 +451,6 @@
           "name": "vm interpreter tests",
           "arguments": [
             "-ndartkb-interpret-linux-${mode}-x64",
-            "--compiler=dartkb",
             "language_2",
             "corelib_2",
             "lib_2",