2.28 is now EOL

Change-Id: If0c04caca66846cae5ab33d21f0ceba77d0c16a6
diff --git a/mbed-tls-nightly-tests.yaml b/mbed-tls-nightly-tests.yaml
index 30e05e4..2d44769 100644
--- a/mbed-tls-nightly-tests.yaml
+++ b/mbed-tls-nightly-tests.yaml
@@ -79,4 +79,3 @@
         cron: |
           H(0-29) 0 * * * %MBED_TLS_BRANCH=development
           H(0-29) 2 * * * %MBED_TLS_BRANCH=mbedtls-3.6
-          H(0-29) 4 * * * %MBED_TLS_BRANCH=mbedtls-2.28