tf-m-split-build: Leave PSA_ARCH_TESTS_VERSION empty

Version of PSA arch tests can be parsed from tf-m-tests. So there is no
need to set a fixed versions value.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I422535f806272f074b838b071c9d8e4af883c14f
diff --git a/tf-m-split-build.yaml b/tf-m-split-build.yaml
index a7f593d..f55e2c5 100644
--- a/tf-m-split-build.yaml
+++ b/tf-m-split-build.yaml
@@ -95,7 +95,7 @@
           default: 'https://git.trustedfirmware.org/mirror/psa-arch-tests.git'
       - string:
           name: 'PSA_ARCH_TESTS_VERSION'
-          default: '5c57920'
+          default: ''
           description: |
             Specify which psa-arch-tests version to use
             refs/heads/<branch_name> : Pick up latest HEAD on branch