Remove perspectives plugin and context.

[linuspahl]

Description

Motivation and Context

• Remove the perspectives switcher from navigation and related UI actions.
• Route to the default welcome page and drop perspective-specific telemetry metadata.
• Clean up perspective context usage, imports, and test mocks.

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have requested a documentation update.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

/prd https://github.com/Graylog2/graylog-plugin-enterprise/pull/13269