// Assuming there is exactly one matching suffix (this will check that)
var suffixToStrip = suffixes.Single(x => myString.EndsWith(x));
// Replace the matching one:
var stringWithoutSuffix = Regex.Replace(myString, "(" +suffixToStrip + ")$", "");
ИЛИ, поскольку вы знаете длину соответствующего суффикса:
// Assuming there is exactly one matching suffix (this will check that)
int trim = suffixes.Single(x => myString.EndsWith(x)).Length;
// Remove the matching one:
var stringWithoutSuffix = myString.Substring(0, myString.Length - trim);