Exclude dlls from build

Hi!
I build my app into apk file and get editor log that makes me kind of dissapoinmented.

There are dependencies in my build:
Mono.Security.dll
System.Core.dll
System.dll
mscorlib.dll
UnityEngine.dll
UnityEngine.Networking.dll
Assembly-Sharp.dll

And I tend to think that by using mscorlib into stripping level could make me free of System.dll.

I use nothing special but only IEnumerators and primitive types.

Could someone suggest what can I do for exlude non-using dlls ?

Thank you!