main/postmarketos-initramfs: don't hard-code libz path

It's the work of the mkinitfs to fetch the library. This was initially introduced in c7c2b2d0 and its use-case is non-existent today as osk-sdl is gone. In addition, the mkinitfs rework Clayton did some time ago correctly detects libraries needed for binaries, exactly so we don't need to pull this in manually!

This should not go in without @calebccff and @craftyguy blessing!