Section 3.2 remove sentence

[Xavier Vilajosana]

15. The last sentence of section 3.2 seems to be a low-level implementation detail that can be omitted.

TW> Editorial. TW> @Editor, please remove TW> In a memory-efficient implementation, TW> scheduled cells can be represented by a circular linked list. TW> Unscheduled cells SHOULD NOT occupy any memory.