Tiled Gallery: Remove invalidating strip attribute from block save

[sirreal]

Adding ?strip=all to the img[src] causes block invalidation. Revert
this invalidating change.

Introduced in #12061, discovered in D26982-code

Testing instructions:

• srcsets from Tiled gallery block: Add srcset to images in the editor #12061 continue to work well
• Older posts are not invalidated by missing strip=all on the src. The console in the editor would show a message like the following when loading an older post:

Block validation: Expected attribute `src` of value `https://example.com/image.jpg?strip=all`, saw `https://example.com/image.jpg`.

Will be handled as part of D26982-code.

Proposed changelog entry for your changes:

None

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: June 4, 2019.
Scheduled code freeze: May 28, 2019

Generated by 🚫 dangerJS against 90689df

[jeherve]

LGTM! 👍

[simison]

Looks good and tested in D26982-code

[simison]

r191914-wpcom