feat(runtime)!: remove filetype.vim (#20428)

Made obsolete by now graduated `filetype.lua` (enabled by default).

Note that changes or additions to the filetype detection still need to
be made through a PR to vim/vim as we port the _logic_ as well as tests.

src/nvim/testdir/test_legacy_filetype.vim

@@ -1,4 +0,0 @@
-let g:do_legacy_filetype = 1
-filetype on
-
-source test_filetype.vim