Revert "fix web break introduced in 38f130071 fix(linux): enable mouse side buttons in remote sessions (#14848)" (#14973)

This reverts commit d5d0b01266.

flutter/lib/models/input_model.dart

@@ -346,7 +346,7 @@ class InputModel {
   /// which runs per-engine, so each isolate registers its own handler tied
   /// to its own set of InputModels.
   static void initSideButtonChannel() {
-    if (!isLinux) return;
+    if (!Platform.isLinux) return;
     if (_sideButtonChannelInitialized) return;
     _sideButtonChannelInitialized = true;