Remove deprecated 'print_emoji_styles' when generating the block editor's iframe assets

[Mamaduka]

PR removes the recently deprecated print_emoji_styles handler when generating the block editor's iframe assets.

Why

Deprecation notice triggered by the callback breaks editor styles when WP_DEBUG is set to true.

Testing Instructions.

1. Enable debugging:

define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', false );
define( 'WP_DEBUG_DISPLAY', true );

2. Open a post or page.
3. Insert a group block.
4. Placeholder styles shouldn't be broken.
5. Run the following script on the DevTools console - wp.data.select( 'core/block-editor' ).getSettings().__unstableResolvedAssets.styles
6. Returned value shouldn't include a deprecation message.

Screenshot

Trac ticket: https://core.trac.wordpress.org/ticket/58775.

[Mamaduka]

Note: Fix here uses conditional check before removing and adding the callback. This differs from a fix that landed in the Gutenberg repo this morning. I'll sync changes back if this gets committed.

[spacedmonkey]

@hellofromtonya Do you want to review as well. I plan to commit once the unit tests pass.

[hellofromtonya]

Testing it now. Give me a few minutes.

[hellofromtonya]

Tested in a default local testing environment:

• Get the deprecation notice before applying this patch.
• After applying the patch, no more deprecation notice ✅

This change resolves the issue. LGTM for commit 👍

[hellofromtonya]

@spacedmonkey LGTM for commit. Thank you everyone for resolving this issue!

[spacedmonkey]

Committed 6a06786