shell: Drop default from --target

Forgotten by 49cb9b7a7b: The default is now set in
get_bitbake_targets.

This fixes spurious overwrites of the target defined in the config file.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/shell.py

@@ -53,8 +53,7 @@ class Shell:
                             help='Config file')
         sh_prs.add_argument('--target',
                             action='append',
-                            help='Select target to build',
-                            default='core-image-minimal')
+                            help='Select target to build')
         sh_prs.add_argument('--skip',
                             help='Skip build steps',
                             default=[])