Remove extraneous dependency from CBL-Android POM

The build process for the 2.8.x versions of Android is including an unnecessary dependency on kotlin-stdlib:1.4.0.

Nothing in the CBL library code actually depends on the kotlin stdlib, and this explicit dependency may conflict with client code Kotlin version.

This problem has been addressed in the Lithium version. The build-script code to manage dependencies explicitly needs to be backported to the Hydrogen release.