Removing check for leading hyphens in .NET FirefoxOptions AddArgument

In prior releases of geckodriver, it was a requirement that all arguments
be preceeded by double hyphens ("--"). Geckodriver has now relaxed that
restriction, and the .NET bindings need to keep up with that change. This
commit removes the check for leading hyphens when calling AddArgument.

dotnet/src/webdriver/Firefox/FirefoxOptions.cs

@@ -76,10 +76,6 @@ public FirefoxOptions()
             : base()
         {
             this.BrowserName = BrowserNameValue;
-            /*capabilityName == IsMarionetteCapability ||
-                            capabilityName == FirefoxLegacyProfileCapability ||
-                            capabilityName == FirefoxLegacyBinaryCapability ||
-                            capabilityName == FirefoxOptionsCapability)*/
             this.AddKnownCapabilityName(FirefoxOptions.FirefoxOptionsCapability, "current FirefoxOptions class instance");
             this.AddKnownCapabilityName(FirefoxOptions.IsMarionetteCapability, "UseLegacyImplementation property");
             this.AddKnownCapabilityName(FirefoxOptions.FirefoxProfileCapability, "Profile property");
@@ -183,14 +179,6 @@ public void AddArguments(IEnumerable<string> argumentsToAdd)
                 throw new ArgumentNullException("argumentsToAdd", "argumentsToAdd must not be null");
             }
 
-            foreach (string argument in argumentsToAdd)
-            {
-                if (!argument.StartsWith("--", StringComparison.OrdinalIgnoreCase))
-                {
-                    throw new ArgumentException(string.Format(CultureInfo.InvariantCulture, "All arguments must start with two dashes ('--'); argument '{0}' does not.", argument), "argumentsToAdd");
-                }
-            }
-
             this.firefoxArguments.AddRange(argumentsToAdd);
         }