Split Build: Clear tf-m-tests defualt version

Auto-parsing tf-m-tests version is also supported in tf-m-split-build
job. So clear the default setting of tf-m-tests version.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I9d5025a0c443be45c486d41162fd79105d596812
diff --git a/tf-m-split-build.yaml b/tf-m-split-build.yaml
index a70081e..a7f593d 100644
--- a/tf-m-split-build.yaml
+++ b/tf-m-split-build.yaml
@@ -83,7 +83,7 @@
           default: 'https://git.trustedfirmware.org/TF-M/tf-m-tests.git'
       - string:
           name: 'TFM_TESTS_REFSPEC'
-          default: 'refs/heads/feature-build-split-v2'
+          default: ''
       - string:
           name: 'TFM_EXTRAS_URL'
           default: 'https://git.trustedfirmware.org/TF-M/tf-m-extras.git'