common: sched_sedf: Remove unused functions

common: sched_sedf: Remove unused functions

Clang 3.5 will fail to compile xen/common/sched_sedf.c because some function
are not used:

sched_sedf.c:141:20: error: unused function 'extraq_add_head' [-Werror,-Wunused-function]
static inline void extraq_add_head(struct vcpu *d, int i)
                       ^
sched_sedf.c:147:20: error: unused function 'extraq_add_tail' [-Werror,-Wunused-function]
static inline void extraq_add_tail(struct vcpu *d, int i)

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>