Remove duplicate xunit dependency declaration

Author: nblumhardt

test/Serilog.Settings.AppSettings.Tests/project.json

@@ -17,7 +17,6 @@
       "dependencies": {
         "Newtonsoft.Json": "7.0.1",
         "Rx-Main": "2.2.5",
-        "xunit": "2.1.0",
         "xunit.runner.visualstudio": "2.1.0",
         "Serilog.Sinks.TextWriter": "2.0.0",
         "Serilog.Sinks.RollingFile": "2.0.0",