Remove unexisting category

Author: ogoffart

qttypes/Cargo.toml

@@ -7,7 +7,7 @@ build = "build.rs"
 description = "Manually maintained buildings for Qt value types"
 readme = "README.md"
 license = "MIT"
-categories = ["api-bindings", "ffi"]
+categories = ["api-bindings"]
 keywords = ["Qt", "QtCore", "QtGui"]
 repository = "https://github.com/woboq/qmetaobject-rs"
 links = "qt"