[native] Remove code support for experimental KonanTargets

ZEPHYR was the first and the last experimental KonanTarget.
As ZEPHYR is now entirely removed from the codebase, the respective code
can be dropped

^KT-64517

native/native.tests/tests/org/jetbrains/kotlin/konan/test/blackbox/support/NativeTestSupport.kt

@@ -177,7 +177,7 @@ internal object NativeTestSupport {
         val nativeHome = getOrCreateTestProcessSettings().get<KotlinNativeHome>()
 
         val distribution = Distribution(nativeHome.dir.path)
-        val hostManager = HostManager(distribution, experimental = false)
+        val hostManager = HostManager()
         val nativeTargets = computeNativeTargets(enforcedProperties, hostManager)
 
         val cacheMode = computeCacheMode(enforcedProperties, distribution, nativeTargets, optimizationMode)

native/native.tests/tests/org/jetbrains/kotlin/konan/test/blackbox/support/settings/SettingsContainers.kt

@@ -71,5 +71,5 @@ internal val Settings.configurables: Configurables
             // Development variant of LLVM is used to have utilities like FileCheck
             propertyOverrides = mapOf("llvmHome.${HostManager.hostName}" to "\$llvm.${HostManager.hostName}.dev")
         )
-        return PlatformManager(distribution, true).platform(get<KotlinNativeTargets>().testTarget).configurables
+        return PlatformManager(distribution).platform(get<KotlinNativeTargets>().testTarget).configurables
     }