Remove FEAT_OSFILETYPE

Feature description from Vim documentation:

NOTE: this code is currently disabled, as the RISC OS implementation was
removed.  In the future this will use the 'filetype' option.

On operating systems which support storing a file type with the file, you can
specify that an autocommand should only be executed if the file is of a
certain type.

The actual type checking depends on which platform you are running Vim
on; see your system's documentation for details.

To use osfiletype checking in an autocommand you should put a list of types to
match in angle brackets in place of a pattern, like this: >

	:au BufRead *.html,<&faf;HTML>  runtime! syntax/html.vim

This will match:

- Any file whose name ends in ".html"
- Any file whose type is "&faf" or "HTML", where the meaning of these types
  depends on which version of Vim you are using.
  Unknown types are considered NOT to match.