remove some dead code. ArgTokens can never be empty, because it is always

remove some dead code.  ArgTokens can never be empty, because it is always
terminated with an EOF token.  The condition it is trying to check for is
handled by this code above.

    // Empty arguments are standard in C99 and supported as an extension in
    // other modes.
    if (ArgTokens.empty() && !Features.C99)
      Diag(Tok, diag::ext_empty_fnmacro_arg);

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@67705 91177308-0d34-0410-b5e6-96231b3b80d8