Remove Sentry settings from application settings

What does this MR do?

new description (same as old description)

This is a follow-up MR for !27091 (merged) and removes Sentry from application settings as announced here

We agreed on the following here with @dbalexandre and @stanhu

1. Remove "Error Reporting and Logging" section;
2. Remove the columns sentry_enabled, sentry_dsn, clientside_sentry_enabled and clientside_sentry_dsn from the application_settings table;
3. Remove Sentry settings from Admin API;

updated description (deprecated)

This is a follow-up MR for !27091 (merged) and sets the Sentry settings within application settings as announced here to read-only instead of removing them as discussed here.

old description:

This is a follow-up MR for !27091 and removes Sentry from application settings as announced [here](https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/22103)

We agreed on the following [here](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27091#note_158941995) with @dbalexandre and @stanhu 

1. Remove "Error Reporting and Logging" section;
1. Remove the columns `sentry_enabled`, `sentry_dsn`, `clientside_sentry_enabled` and `clientside_sentry_dsn` from the `application_settings` table;
1. Remove Sentry settings from Admin API;

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team