Remove Xcode 15 support from test script

The `test.sh` script is updated to remove the logic for Xcode 15, which is no longer supported.

Author: google-labs-jules[bot]

scripts/test.sh

@@ -28,8 +28,6 @@ if [[ "$xcode_major" -ge 26 ]]; then
   iphone_version="17"
 elif [[ "$xcode_major" -ge 16 ]]; then
   iphone_version="16"
-elif [[ "$xcode_major" -ge 15 ]]; then
-  iphone_version="15"
 else
   echo "Unsupported Xcode version $xcode_version; exiting." 1>&2
   exit 1