ipv6: remove unused variables in esp6

Resolves warnings:
net/ipv6/esp6.c: In function ‘esp_ssg_unref’:
net/ipv6/esp6.c:121:10: warning: variable ‘seqhi’ set but not used [-Wunused-but-set-variable]
net/ipv6/esp6.c: In function ‘esp6_output_head’:
net/ipv6/esp6.c:227:21: warning: variable ‘esph’ set but not used [-Wunused-but-set-variable]

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>