Remove unused _layouts/category.html (#147)

Added in #144 with the intent that am2pla-site would emit
"layout: category" frontmatter on generated pages so the layout would
inject the search box and shared footer. The subsequent rewrite of
am2pla-site on AM main (ivan-hc/AM commits 209b1b63e, eba5430fc,
0226034b5) took a different approach: the script emits the search-box
HTML inline via an ADD_SEARCH_BAR function and writes the footer
inline too. None of the generated .md files declare a layout, so they
fall through to the Primer theme's default and this file has no
effect on the rendered site.

No replacement needed — the inline emission from am2pla-site fully
covers what the layout used to handle.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: andy5995