Revert the addition of build dependency in commit 2d1694ff7
to a non-existent host build of zlib.

The host build of zlib was removed already in April 2018 by
8dcd941d8b (diff-1ed408c61d79f9c6c5d197333e94ce8d)
which made zlib a build tool defined in /tools

The newly introduced build dependency causes always a warning like:
   WARNING: Makefile 'package/feeds/packages/postgresql/Makefile'
   has a build dependency on 'zlib/host', which does not exist

Not sure what was the error that 2d1694ff7 tried to fix,
but reference to a non-existent host build is not the solution.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>