GNU sed has a suitable addressing mode:
sed -n '1~2!p' file
which means, starting from line 1, and with step 2, print all other lines.
Equivalently, you can drop the -n, and delete matching lines:
sed '1~2d'
It can also be done using awk:
awk 'NR%2==0' file
(Whenever line number is a multiple of 2, print the line)