Remove parallax support from Prettyblocks

[CySSoO]

Motivation

• Remove parallax support from all Prettyblock types to simplify block rendering and avoid use of background-attachment: fixed and parallax-specific markup and styles.

Description

• Removed the 'parallax' checkbox fields from Prettyblocks schema definitions in src/Service/EverblockPrettyBlocks.php so the option is no longer present in block settings.
• Simplified templates in views/templates/hook/prettyblocks/prettyblock_cover.tpl, prettyblock_text_and_image.tpl, and prettyblock_cta.tpl to stop emitting parallax-specific markup and to always render images using <picture><img> elements.
• Removed parallax-specific CSS rules from views/css/everblock.css, including .everblock-parallax blocks and .prettyblock-cover-item--parallax rules.
• Cleaned up conditional background-attachment and inline parallax styles in cover/CTA/templates to use standard background/image rendering.

Testing

• No automated tests were executed for this change.

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Codex Task