Move ftplugin to after/ftplugin

Make my own settings have the final say on how a filetype should behave,
instead of some 3rd party plugins.

after/ftplugin/yaml.vim

@@ -2,3 +2,8 @@ set tabstop=2       " number of visual spaces per TAB
 set softtabstop=2   " number of spaces in tab when editing
 set shiftwidth=2    " number of spaces to use for autoindent
 set expandtab       " expand tab to spaces so that tabs are spaces
+
+" Turn off syntax highlighting for large YAML files.
+if line('$') > 500
+  setlocal syntax=OFF
+endif

ftplugin/yaml.vim

@@ -1,4 +0,0 @@
-" Turn off syntax highlighting for large YAML files.
-if line('$') > 500
-  setlocal syntax=OFF
-endif