xfrm: Remove duplicate assignment

The statement in the switch is repeated with the statement at the
beginning of the while loop, so this statement is meaningless.

The clang_analyzer complains as follows:

net/xfrm/xfrm_policy.c:3392:2 warning:

Value stored to 'exthdr' is never read

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: luo penghao <luo.penghao@zte.com.cn>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>