MCP still uses old plurality forms of language variables

When the new plurality feature was added to the language system (PHPBB3-10345, it removed a few language variables. Some of those were still explicitly called in the template files, resulting in them not being converted to their appropriate language strings. The only keys that appear to still be in use are:

• PM_REPORTS_ZERO_TOTAL
• REPORTS_ZERO_TOTAL
• UNAPPROVED_POSTS_ZERO_TOTAL
  These are used in mcp_front.html.