Removes option to delete a signal definition

[mvilanova]

Summary of Changes

1. Backend - service.py and views.py:

   • Removed the delete function in service.py, which previously handled deletion of signal definitions.
   • Deleted the delete_signal endpoint in views.py, including the associated permission check for deleting signals.
   • These changes eliminate the capability to delete signals from the backend.

2. Frontend - Vue Components:

   • DeleteDialog.vue: Removed entirely. This file previously handled the delete confirmation dialog for signals.
   • Table.vue:
     • Removed <delete-dialog /> component reference from the template.
     • Deleted the delete action from the item menu, removing the "Delete" option for signals in the UI.
     • Updated components import to exclude DeleteDialog.

These modifications effectively remove the signal deletion functionality from both the backend and frontend.