zlib: remove ldconfig call from install-libs

zlib: remove ldconfig call from install-libs

* it's called from install-libs target and when /etc/ld.so.cache is writeable by user running bitbake
  then it creates invalid cache (in my case libstdc++.so cannot be found after building zlib(-native)
  and I have to call touch */libstdc++.so && /sbin/ldconfig to fix it

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>