Revert "MSC3575 (Sliding Sync) add well-known proxy support (#12307)"

src/MatrixClientPeg.ts

src/SlidingSyncManager.ts

src/components/views/dialogs/SlidingSyncOptionsDialog.tsx

src/i18n/strings/en_EN.json

src/settings/Settings.tsx

src/settings/controllers/SlidingSyncController.ts

test/SlidingSyncManager-test.ts