[test] Remove redundant gen_snapshot passing in crossword builder

The gen_snapshot path passing was introduced in https://dart-review.googlesource.com/c/sdk/+/107644.
However, it was made redundant in https://dart-review.googlesource.com/c/sdk/+/114850 which selects gen_snapshot based on --builder_tag crossword.

See discussion at https://dart-review.googlesource.com/c/sdk/+/133521/1/pkg/test_runner/lib/src/compiler_configuration.dart#694.

Change-Id: Ib7cf40dc474a681ac177ecbb93f2fbfc3d80e50d

Cq-Include-Trybots: luci.dart.try:vm-kernel-precomp-android-release-arm_x64-try,vm-kernel-precomp-linux-debug-simarm_x64-try,vm-kernel-precomp-linux-release-simarm_x64-try
Change-Id: Ib7cf40dc474a681ac177ecbb93f2fbfc3d80e50d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/133560
Reviewed-by: Martin Kustermann <kustermann@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>

tools/bots/test_matrix.json

@@ -3262,8 +3262,7 @@
         {
           "name": "vm tests",
           "arguments": [
-            "-ndartkp-${system}-${mode}-simarm-crossword",
-            "--gen-snapshot=${build_root}/gen_snapshot"
+            "-ndartkp-${system}-${mode}-simarm-crossword"
           ],
           "fileset": "vm-kernel",
           "shards": 10