string.ReplaceWithEmpty(string toReplace)

[Test]
public void ShouldReplaceWithEmpty()
{
const string arg = "This is a string";

Assert.AreEqual("This is a", arg.ReplaceWithEmpty(" string"));
}

public static string ReplaceWithEmpty(this string arg, string toReplace)
{
return arg.Replace(toReplace, string.Empty);
}