Remove appcode checker

As discussed in https://github.com/owncloud/core/issues/9574 the appcodechecker can be easily bypassed and is therefore not really useful but creates some bugs.
This is not something we can fix since automatically detecting malicious PHP code is impossible with our current application design.

In my opinion we should drop this code, it gives a wrong sense of security and if we want to check for malicious app codes we have to do this in the appstore and not on every oC instance. (e.g. by reviewing the source code)

@PVince81 @georgehrke @karlitschek