Remove the use of -developer-build from the INTEGRITY instructions

The -developer-build option is only intended for use by people
developing Qt and it will result in a configuration that is sub-optimal
for releasing Qt applications, by extra run-time checks being performed
and symbols being exported that are not part of the public ABI.

Change-Id: If83147562e3d0e6daf1039d89bb115c25ec717ba
Reviewed-by: Janne Koskinen <janne.p.koskinen@qt.io>
Reviewed-by: Kimmo Ollila <kimmo.ollila@qt.io>
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>