Remove explicit Kotlin standard library dependency

The Kotlin Gradle plugin adds the version of kotlin-stdlib that matches itself, so declaring the dependency explicitly is redundant. Removes org.jetbrains.kotlin:kotlin-stdlib and its -jdk7, -jdk8 and -common variants, written either as coordinates or with the Kotlin DSL kotlin("stdlib") helper, from a project that applies a Kotlin plugin. Only implementation declarations are removed, because that is the one configuration the Kotlin plugin contributes the standard library to: a declaration on api is consumer-visible through the published compile scope, and one on a configuration of the project's own has nothing putting it back. A declaration is kept when it pins a version newer than the plugin's, since removing it would quietly downgrade the standard library; when the plugin predates 1.4, the first version that adds the standard library itself; and when kotlin.stdlib.default.dependency=false turns that default off, leaving the declaration as the only thing providing it. See the Gradle best practice.