Revert "Re-enable selection UI for TEST_BRANCH"

This reverts commit 514d5f79b8809c76635a3b4b030a03513fc4e82e.

Reason for revert: Bug in Git parameter plugin

Change-Id: I1535db76ded12a863d7373aaa6caf33c72bfa33e
diff --git a/mbedtls-release-ci-testing.yaml b/mbedtls-release-ci-testing.yaml
index 8f1701b..e22cfd9 100644
--- a/mbedtls-release-ci-testing.yaml
+++ b/mbedtls-release-ci-testing.yaml
@@ -66,13 +66,11 @@
         default: false
         description: Push head to coverity branch, triggering a scan (only done for the development branch)
         name: PUSH_COVERITY
-    - git-parameter:
+    - string:
         name: TEST_BRANCH
         description: Branch or tag in https://github.com/Mbed-TLS/mbedtls-test.git (forks are not supported)
-        type: PT_BRANCH
-        quickFilterEnabled: true
-        defaultValue: main
-        selectedValue: DEFAULT
+        default: main
+        trim: true
     - string:
         name: TEST_PASS_EMAIL_ADDRESS
         description: E-mail addresses (delimited by ,) that a report should be sent to when the test run passes