vim: Don't set foldmethod in the syntax file either

We shouldn't be setting any settings in the syntax file. Better to put
them in the ftplugin, where they won't be pulled in by :syn-include and
can be cleaned up when changing the filetype.
auto merge of #16527 : kballard/rust/vim_fold, r=chris

We shouldn't be setting any settings in the syntax file. Better to put
them in the ftplugin, where they won't be pulled in by :syn-include and
can be cleaned up when changing the filetype.