[fractal] fractal-matrix-api: Drop a redundant conversion

[Alexandre Franke <afranke src gnome org>]

commit f701ea559c0ab9dde0a27b18c98711e203d77ad4
Author: Zeeshan Ali <zeenix collabora co uk>
Date:   Wed Dec 12 23:05:44 2018 +0100

    fractal-matrix-api: Drop a redundant conversion
    
    This fixes this errors from clippy:
    
    ```
    warning: identical conversion
      --> fractal-matrix-api/src/backend/sync.rs:53:32
       |
    53 |         params.push(("filter", String::from(filter)));
       |                                ^^^^^^^^^^^^^^^^^^^^ help: consider removing `String::from()`: 
`filter`
       |
       = help: for further information visit 
https://rust-lang-nursery.github.io/rust-clippy/master/index.html#identity_conversion
    ```
    
    Related: #370

 fractal-matrix-api/src/backend/sync.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/fractal-matrix-api/src/backend/sync.rs b/fractal-matrix-api/src/backend/sync.rs
index a99f131..f043582 100644
--- a/fractal-matrix-api/src/backend/sync.rs
+++ b/fractal-matrix-api/src/backend/sync.rs
@@ -50,7 +50,7 @@ pub fn sync(bk: &Backend, new_since: Option<String>, initial: bool) -> Result<()
             "event_fields": ["type", "content", "sender", "origin_server_ts", "event_id", "unsigned"]
         }}"#, globals::PAGE_LIMIT);
 
-        params.push(("filter", String::from(filter)));
+        params.push(("filter", filter));
         params.push(("timeout", String::from("0")));
         timeout = 0;
     }