Version Control of Snippets: Remove snippet blob search

This is a follow up issue of #199220 (closed)

It's decided to remove snippet blob search functionality. However since we have to maintain backward API compatibility, we can only do so after GitLab 13.0.

We will need to remove the remaining references to searching snippet blob content, and also remove the ability to search snippet file names now that they are also in the snippet repositories.

Finally, we should also update the UI from Titles and Filenames to Titles and Descriptions

MR Breakdown

@fjsanpedro:

• 1 MR to update the views
• 1 MR to API endpoints
• 1 MR to update the controllers and all the remaining search code we have scoped to the snippet content.

We will need some small investigation to remove those methods or classes we don't need