Stop using S3 CocoaPods Specs mirror (#30265)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/30265

The CocoaPods Specs mirror provided by Circle CI is no longer supported, and is no longer necessary, as of CocoaPods v1.8. We are already using the CDN in the Podfile, so the step may be removed from the Circle CI config.

Reference: https://circleci.com/docs/2.0/testing-ios/#optimizing-cocoapods
Changelog: [Internal]

Reviewed By: fkgozali

Differential Revision: D24601585

fbshipit-source-id: c794102cf573e0cf47ae936bf3c1eb386e089315

.circleci/config.yml

@@ -407,10 +407,6 @@ jobs:
                 name: Set USE_FRAMEWORKS=1
                 command: echo "export USE_FRAMEWORKS=1" >> $BASH_ENV
 
-      - run:
-          name: Fetch CocoaPods Specs
-          command: |
-            curl https://cocoapods-specs.circleci.com/fetch-cocoapods-repo-from-s3.sh | bash -s cf
       - run:
           name: Setup the CocoaPods environment
           command: pod setup
@@ -421,10 +417,6 @@ jobs:
                 name: Generate RNTesterPods Workspace
                 command: cd packages/rn-tester && bundle exec pod install --verbose
 
-      - run:
-          name: Generate RNTesterPods Xcode Workspace
-          command: pushd packages/rn-tester && bundle exec pod install --verbose && popd
-
       # -------------------------
       # Runs iOS unit tests
       - when: