Remove merch_sidebar partial from marketing blog to sanitize for Netlify CMS

Why is this change being made?

This MR removes the <%= partial "includes/blog/blog-merch-sidebar-dynamic" %> from all marketing blog posts. This is done to sanitize the marketing blog posts markdown for a new template being implement with Netlify CMS.

We are keeping the merch_sidebar: frontmatter intact so we can re-implement a sidebar on the next iteration of the blog template. The merch_sidebar content will still be there... we just wont be using it in the first iteration.

Closes https://gitlab.com/gitlab-com/marketing/inbound-marketing/growth/-/issues/1069

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