Remove prepend_attachment from filters run on content for post template parts

[johnstonphilip]

This PR is a follow-up to PR #20343 for issue #20342

Description

The need for all of the filters which run on the_content was discussed in #20343, and after testing each one, I discovered that only prepend_attachment appears to be unneeded in the post_template_part context. Since prepend_attachment only runs on post_type(s) set to be attachment, it is unnecessary in the this context. Because of that, I am removing it here.

However, all other filters applied have been tested and I found them to be necessary. See my discoveries in this comment:
#20343 (comment)

How has this been tested?

1. I created a block based theme using a header.html template part, placed blocks, shortcodes, multi-line breaks, and more inside the header.html file.
2. I viewed a page and confirmed that all of those things continued to work as expected.

Types of changes

Bug fix.

Checklist:

• [✅] My code is tested.
• [✅] My code follows the WordPress code style.
• [✅] My code follows the accessibility standards.
• [✅] My code has proper inline documentation.
• [✅] I've included developer documentation if appropriate.
• [✅] I've updated all React Native files affected by any refactorings/renamings in this PR.