e1000e: Remove redundant statement

This assignment statement is meaningless, because the statement
will execute to the tag "set_itr_now".

The clang_analyzer complains as follows:

drivers/net/ethernet/intel/e1000e/netdev.c:2552:3 warning:

Value stored to 'current_itr' is never read.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: luo penghao <luo.penghao@zte.com.cn>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>