Assembler: Issues on post-meta pattern and template part

[miksansegundo]

What

There are issues with the post-meta pattern and its template part because they were copied from mpho theme

1. The post-meta template part uses the slug mpho/post-meta, which is wrong because it should be assembler/post-meta. See

   themes/assembler/parts/post-meta.html

   Line 1 in b689c25

   <!-- wp:pattern {"slug":"mpho/post-meta"} /-->

2. The pattern should include a comment with Inserter: no because it's not meant to be listed in the inserter. @alaczek can you confirm this?
3. The pattern uses the wrong pattern category post meta. This category is not registered and is not meant to be used here.

Why

The first issue is the most important because the template is broken.
The second issue shows that pattern in the editor inserter as "uncategorized".
The third issue could fix the second issue if we use a valid category but I believe that pattern should not be in the inserter.

How

1. use assembler/post-meta as the slug
2. add the comment Inserter: no
3. remove the comment Categories: post meta

[richtabor]

In #7538 I've removed the post meta (and a few other templates/parts) that are no longer necessary.

[alaczek]

It looks like @richtabor already addressed it! That PR should be good to merge.