[xcassets] Removed code adding resources to the Pods project. This was the cause for (null) references in the generated Pods project.

(this was an attempt to get rid of (null) references in the generated Xcode project; it finally wasn't the cause, but keeping the test for safety)

This reverts commit f4c64317.

Even that could be fixed by appropriate configuration, there is just no case, where it would make sense to have an asset catalog inside of an resource bundle.

[xcassets] Remove from the PBXBuildPhase all resources of pods no longer integrated with a native target
(WIP, XcodeProj bug to investigate)

[IntegrationSpec] Workaround rebuild inconsistency

[IntegrationSpec] Add missing specs to integration.rb

Do not pass code-sign arguments for OS X targets.

Fixes #3310

Closes #3344.

Closes #3345.