matrix-react-sdk/src/customisations/Security.ts is no longer used and should be removed

[richvdh]

matrix-org/matrix-react-sdk#12342 removed the code that called matrix-react-sdk/src/customisations/Security.ts, so in theory we can just rip it out.

The problem is that it is still used as the example in https://github.com/element-hq/element-web/blob/develop/docs/customisations.md. So we'd either need to update the docs, or assume that nobody needs any of the "customisations" stuff any more and remove the documentation altogether.