cleanup: delete custom ethers fallback provider

Deletes a no longer used fallback provider that was
mostly copy pasted from ethers v5. There was no good
way to specify the headers and we needed to do so,
so we used this client to ensure that headers can
be passed. It is no longer used so we can safely delete
it.

packages/core-utils/src/external/ethers/fallback-provider.ts

-/**
- * Provider Utilities
- */
-
-import {
-  Provider,
-  StaticJsonRpcProvider,
-  FallbackProvider as EthersFallbackProvider,
-} from '@ethersproject/providers'
-import { ConnectionInfo } from '@ethersproject/web'
-
-export interface HttpHeaders {
-  [key: string]: string
-}
-
-// Copied from @ethersproject/providers since it is not
-// currently exported
-export interface FallbackProviderConfig {
-  // The Provider
-  provider: Provider
-  // The priority to favour this Provider; higher values are used first
-  priority?: number
-  // Timeout before also triggering the next provider; this does not stop
-  // this provider and if its result comes back before a quorum is reached
-  // it will be incorporated into the vote
-  // - lower values will cause more network traffic but may result in a
-  //   faster retult.
-  stallTimeout?: number
-  // How much this provider contributes to the quorum; sometimes a specific
-  // provider may be more reliable or trustworthy than others, but usually
-  // this should be left as the default
-  weight?: number
-}
-
-export const FallbackProvider = (
-  config: string | FallbackProviderConfig[],
-  headers?: HttpHeaders
-) => {
-  const configs = []
-  // Handle the case of a string of comma delimited urls
-  if (typeof config === 'string') {
-    const urls = config.split(',')
-    for (const [i, url] of urls.entries()) {
-      const connectionInfo: ConnectionInfo = { url }
-      if (typeof headers === 'object') {
-        connectionInfo.headers = headers
-      }
-      configs.push({
-        priority: i,
-        provider: new StaticJsonRpcProvider(connectionInfo),
-      })
-    }
-    return new EthersFallbackProvider(configs)
-  }
-
-  return new EthersFallbackProvider(config)
-}