Remove MainThread annotation from runtime

It is unused. Moreover, nothing in the runtime should rely on the concept of a global main thread anyway since it is a concept that only exists on certain platforms.

Test: ./gradlew :compose:runtime:runtime:clean :compose:runtime:runtime:build -Pandroidx.compose.multiplatformEnabled=true
Change-Id: I9d33d613063a2467444bb7ae34496669135ed081
diff --git a/compose/runtime/runtime/src/androidMain/kotlin/androidx/compose/runtime/ActualAndroid.kt b/compose/runtime/runtime/src/androidMain/kotlin/androidx/compose/runtime/ActualAndroid.kt
index 1aab3c2..0043c50 100644
--- a/compose/runtime/runtime/src/androidMain/kotlin/androidx/compose/runtime/ActualAndroid.kt
+++ b/compose/runtime/runtime/src/androidMain/kotlin/androidx/compose/runtime/ActualAndroid.kt
@@ -35,8 +35,6 @@
     }
 }
 
-internal actual typealias MainThread = androidx.annotation.MainThread
-
 internal actual typealias CheckResult = androidx.annotation.CheckResult
 
 /**
diff --git a/compose/runtime/runtime/src/commonMain/kotlin/androidx/compose/runtime/Expect.kt b/compose/runtime/runtime/src/commonMain/kotlin/androidx/compose/runtime/Expect.kt
index 2fa9cba..e363360 100644
--- a/compose/runtime/runtime/src/commonMain/kotlin/androidx/compose/runtime/Expect.kt
+++ b/compose/runtime/runtime/src/commonMain/kotlin/androidx/compose/runtime/Expect.kt
@@ -41,14 +41,6 @@
 }
 
 @MustBeDocumented
-@Retention(AnnotationRetention.BINARY)
-@Target(
-    AnnotationTarget.FUNCTION,
-    AnnotationTarget.CONSTRUCTOR
-)
-expect annotation class MainThread()
-
-@MustBeDocumented
 @Retention(AnnotationRetention.SOURCE)
 @Target(
     AnnotationTarget.FUNCTION,
diff --git a/compose/runtime/runtime/src/desktopMain/kotlin/androidx/compose/runtime/ActualDesktop.kt b/compose/runtime/runtime/src/desktopMain/kotlin/androidx/compose/runtime/ActualDesktop.kt
index 9a4ddf3..3e5c529 100644
--- a/compose/runtime/runtime/src/desktopMain/kotlin/androidx/compose/runtime/ActualDesktop.kt
+++ b/compose/runtime/runtime/src/desktopMain/kotlin/androidx/compose/runtime/ActualDesktop.kt
@@ -87,7 +87,6 @@
 }
 
 // TODO(igotti): do we need actual processing for those?
-actual annotation class MainThread()
 actual annotation class CheckResult(actual val suggest: String)
 
 /**