Social: Remove defaulting to Twitter in share log

[gmjuhasz]

Fixes https://github.com/Automattic/jetpack-reach/issues/649

Proposed Changes

• Removed defaulting to twitter if there is no icon
• Added an unlink icon if we cannot retrieve the service icon

Testing Instructions

• Go to http://calypso.localhost:3000/posts/<your_site>
• Reshare a published post to a connection
• Remove that connection
• In the log it should have a Twitter icon on trunk, but a disconnected icon with this PR

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-122695

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-122695&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-122695&env=a8c-for-agencies

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~208 bytes added 📈 [gzipped])

name          parsed_size           gzip_size
posts-custom       +754 B  (+0.1%)     +208 B  (+0.1%)
posts              +754 B  (+0.1%)     +208 B  (+0.1%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Async-loaded Components (~229 bytes added 📈 [gzipped])

name                      parsed_size           gzip_size
async-load-design-blocks       +767 B  (+0.0%)     +229 B  (+0.0%)

React components that are loaded lazily, when a certain part of UI is displayed for the first time.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• notifications
• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug fix/social-twitter-default-icon on your sandbox.