Set default refspec value as NULL for dependency repos

TF-M dependency versions can be automatically parsed by CI scriptsg from
CMake system now. So keep default refspec value as NULL to use the TF-M
specified dependency version by default.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I91c4ed7f5a4effb154054f8aeb43f6dccf0ecd9b
diff --git a/tf-m-lava-submit.yaml b/tf-m-lava-submit.yaml
index 402065e..7653924 100644
--- a/tf-m-lava-submit.yaml
+++ b/tf-m-lava-submit.yaml
@@ -75,7 +75,7 @@
           default: 'master'
       - string:
           name: 'MBEDTLS_VERSION'
-          default: 'mbedtls-3.4.0'
+          default: ''
           description: |
             Specify which MBEDTLS version to use
             refs/heads/<branch_name> : Pick up latest HEAD on branch