Fix multiple diagnostic virtual text per-line of same severity in vim

Remove the prop_remove just before prop_add in place_virtual_text, since
this function is called after clear_virtual_text is called to clear the
previously added virtual text.

That way, if there are multiple virtual text to place on the same line
with the same severity, the following ones don't replace the former
ones.  Besides, the branch for neovim wasn't removing any property, so
this should make it consistent with neovim.

autoload/lsp/internal/diagnostics/virtual_text.vim

@@ -180,7 +180,6 @@ function! s:place_virtual_text(server, diagnostics_response, bufnr) abort
             " anymore due to async processing, just skip such diagnostics
             if l:line <= l:linecount
                 let l:type = 'vim_lsp_' . l:name . '_virtual_text'
-                call prop_remove({'all': v:true, 'type': l:type, 'bufnr': a:bufnr}, l:line)
                 call prop_add(
                 \ l:line, 0,
                 \ {