Cleanup backendMapping in history.tsx

[raj-manvar]

Currently the history.tsx which is for rendering the Query history page has backendMapping defined at https://github.com/trinodb/trino-gateway/blob/main/webapp/src/components/history.tsx#L18

It stores mapping between:
backend name -> backend proxy URL
backend proxy URL -> backend name
backend external Url -> backend name.

From usage, only the ProxyUrl -> backend Name mapping is used while displaying the backend name in the list of query history, rest mapping can be cleaned up.

Originally discussed at #693 (comment)

[raj-manvar]

Will be resolved in #731