Allow the time to read block to be inserted multiple times

[talldan]

What?

In passing, I noticed that the Time to Read block can't be inserted in a post more than once.

In this PR I'm removing the restriction. I'm not sure if there's a reason I'm missing for keeping multiple: false.

Why?

While it's likely the user would only want the block once, I don't think there's any need to impose the restriction.

How?

Remove the line in the block.json

Testing Instructions

1. Add a time to read block to a post
2. Try adding another time to read block, it should work.

Screenshots or screencast

[github-actions]

Flaky tests detected in 0883c6a.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4487868973
📝 Reported issues:

• [Flaky Test] should be able to create an hierarchical post without title support #36939 in specs/editor/plugins/custom-post-types.test.js
• [Flaky Test] should insert elements from multiple completers in a single block #47916 in /test/e2e/specs/editor/various/autocomplete-and-mentions.spec.js
• [Flaky Test] should show the edited content after revert and clicking undo in the notice #41489 in /test/e2e/specs/site-editor/template-revert.spec.js

[fabiankaegy]

Yeah especially when you want to use the block once in the main post content and then again in a query loop template we need to be able to insert it more than once.

(I still think #38958 would make sense for this at some point :) )

[t-hamano]

LGTM! I can't remember why I set multiple to false when I created this block in #43403, but I see no reason to limit it 😅

[aristath]

LGTM!
The PR needs to be rebased because there are currently some conflicts, but other than that it's good to merge 👍

[talldan]

Thanks for the quick reviews ❤️