Revert "refactor: Execute autocmd in group scope"

This reverts commit 18ed14a.

Do not need "augroup" for "ftdetect", see :h ftdetect, and
"filetypedetect" may is a autocmd group name.

Author: StarryLeo

ftdetect/markdown.vim

@@ -1,12 +1,8 @@
-augroup VimMarkdown
-    autocmd!
-augroup END
-
 if !has('patch-7.4.480')
     " Before this patch, vim used modula2 for .md.
-    au! VimMarkdown filetypedetect BufRead,BufNewFile *.md
+    au! filetypedetect BufRead,BufNewFile *.md
 endif
 
 " markdown filetype file
-au VimMarkdown BufRead,BufNewFile *.{md,mdx,mdown,mkd,mkdn,markdown,mdwn} setfiletype markdown
-au VimMarkdown BufRead,BufNewFile *.{md,mdx,mdown,mkd,mkdn,markdown,mdwn}.{des3,des,bf,bfa,aes,idea,cast,rc2,rc4,rc5,desx} setfiletype markdown
+au BufRead,BufNewFile *.{md,mdx,mdown,mkd,mkdn,markdown,mdwn} setfiletype markdown
+au BufRead,BufNewFile *.{md,mdx,mdown,mkd,mkdn,markdown,mdwn}.{des3,des,bf,bfa,aes,idea,cast,rc2,rc4,rc5,desx} setfiletype markdown