Remove duplicated keys

In the images.yml file we have a duplicate key with the same translation,
so we can remove it.

In the admin.yml file we have a duplicate key with different translation.

This translation is only used in 2 places in the application:
- In the administration table for collaborative legislation proposal index.
- In the <% provide :title do %> of the same page.

In this case I have no preference which one to keep.

So if I have to decide, although it is true that now is being applied the second
translation (Title), I think it makes more sense to use the first (Proposals).

In the admin table it makes sense to use either "Title" or "Proposals", but in
the page title it seems to make more sense to use "Proposals" instead of "Title".

config/locales/en/admin.yml

@@ -619,7 +619,6 @@ en:
           title: Proposals
           back: Back
           id: Id
-          title: Title
           supports: Total supports
         form:
           custom_categories: Categories

config/locales/en/images.yml

@@ -7,7 +7,6 @@ en:
       delete_button: Remove image
       note: "You can upload one image of following content types: %{accepted_content_types}, up to %{max_file_size} MB."
       add_new_image: Add image
-      title_placeholder: Add a descriptive title for the image
     actions:
       destroy:
         notice: Image was deleted successfully.