Remove stray diagnostics

Author: AnaCoda

src/D2L.Security.OAuth2/Validation/AccessTokens/AccessTokenValidator.cs

@@ -8,7 +8,6 @@
 using D2L.Security.OAuth2.Validation.Exceptions;
 using D2L.Services;
 using D2L.CodeStyle.Annotations;
-using System.Diagnostics;
 
 #if DNXCORE50
 using System.IdentityModel.Tokens.Jwt;