Enshrine that all ToolchainContexts in a toolchain collection have the same target platform so the toolchain collection should be able to directly access it.

Along the way, rm an unused method.

PiperOrigin-RevId: 333365303
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java b/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java
index 99f52d9..47f687c 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java
@@ -1219,16 +1219,11 @@
   }
 
   public boolean targetPlatformHasConstraint(ConstraintValueInfo constraintValue) {
-    if (toolchainContexts == null
-        || toolchainContexts.getDefaultToolchainContext().targetPlatform() == null) {
+    if (toolchainContexts == null || toolchainContexts.getTargetPlatform() == null) {
       return false;
     }
     // All toolchain contexts should have the same target platform so we access via the default.
-    return toolchainContexts
-        .getDefaultToolchainContext()
-        .targetPlatform()
-        .constraints()
-        .hasConstraintValue(constraintValue);
+    return toolchainContexts.getTargetPlatform().constraints().hasConstraintValue(constraintValue);
   }
 
   public ConstraintSemantics<RuleContext> getConstraintSemantics() {