[K/N][build] Build scripts cleanup

* Removes unused dependencies
* Cleanup code
* Remove benchmark related sources

kotlin-native/build-tools/build.gradle.kts

@@ -28,7 +28,6 @@ val rootProperties = Properties().apply {
 }
 
 val kotlinVersion = project.bootstrapKotlinVersion
-val slackApiVersion: String by rootProperties
 val metadataVersion: String by rootProperties
 
 group = "org.jetbrains.kotlin"
@@ -64,8 +63,6 @@ dependencies {
         }
     }
 
-    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.5.0")
-
     api(project(":native:kotlin-native-utils"))
     api(project(":kotlin-native-shared"))
     api(project(":kotlinx-metadata-klib"))