vim: Stop setting conceallevel in the syntax file

We shouldn't be setting conceallevel in the syntax file. Besides not
being able to undo this if we switch to another syntax later, it also
interferes with embedding rust in other filetypes (such as markdown).

Instead, set it in the ftplugin, where it belongs.
auto merge of #16486 : kballard/rust/vim_conceal, r=chris

We shouldn't be setting conceallevel in the syntax file. Besides not
being able to undo this if we switch to another syntax later, it also
interferes with embedding rust in other filetypes (such as markdown).

Instead, set it in the ftplugin, where it belongs.