[fractal] Backend: Cairo is unused, don't depend on it

[Julian Sparber <jsparber src gnome org>]

commit a87d712fe4854d012f30ee7edd8997fda19b1129
Author: Alejandro Domínguez <adomu net-c com>
Date:   Sat Dec 29 22:10:18 2018 +0100

    Backend: Cairo is unused, don't depend on it

 Cargo.lock                      | 1 -
 fractal-matrix-api/Cargo.toml   | 4 ----
 fractal-matrix-api/src/error.rs | 2 --
 3 files changed, 7 deletions(-)
---
diff --git a/Cargo.lock b/Cargo.lock
index 01b92914..b9599ccd 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -476,7 +476,6 @@ dependencies = [
 name = "fractal-matrix-api"
 version = "4.0.0"
 dependencies = [
- "cairo-rs 0.5.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "chrono 0.4.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "glib 0.6.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "lazy_static 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
diff --git a/fractal-matrix-api/Cargo.toml b/fractal-matrix-api/Cargo.toml
index cc4c5af7..f50f3186 100644
--- a/fractal-matrix-api/Cargo.toml
+++ b/fractal-matrix-api/Cargo.toml
@@ -28,10 +28,6 @@ tree_magic = "0.2.1"
 url = "1.7.2"
 urlencoding = "1.0.0"
 
-[dependencies.cairo-rs]
-version = "0.5.0"
-features = ["png"]
-
 [dependencies.chrono]
 version = "0.4.6"
 features = ["serde"]
diff --git a/fractal-matrix-api/src/error.rs b/fractal-matrix-api/src/error.rs
index b344b560..30ba7ba4 100644
--- a/fractal-matrix-api/src/error.rs
+++ b/fractal-matrix-api/src/error.rs
@@ -23,8 +23,6 @@ impl From<reqwest::Error> for Error {
 derror!(url::ParseError, Error::BackendError);
 derror!(io::Error, Error::BackendError);
 derror!(regex::Error, Error::BackendError);
-derror!(cairo::Status, Error::BackendError);
-derror!(cairo::IoError, Error::BackendError);
 derror!(glib::Error, Error::BackendError);
 derror!(SystemTimeError, Error::BackendError);