Remove MainNinjaFile()

It was the same as SoongNinjaFile, except it used
shared.JoinPath instead of filepath.Join, which
doesn't matter here because the second argument
is never an absolute path.

Test: Presubmits
Change-Id: I95e64a8b8971bd168a1dc6670ae80f4d336045b5
diff --git a/ui/build/soong.go b/ui/build/soong.go
index 617d293..a0f223b 100644
--- a/ui/build/soong.go
+++ b/ui/build/soong.go
@@ -170,7 +170,7 @@
 	ctx.BeginTrace(metrics.RunSoong, "blueprint bootstrap")
 	defer ctx.EndTrace()
 
-	mainSoongBuildExtraArgs := []string{"-o", config.MainNinjaFile()}
+	mainSoongBuildExtraArgs := []string{"-o", config.SoongNinjaFile()}
 	if config.EmptyNinjaFile() {
 		mainSoongBuildExtraArgs = append(mainSoongBuildExtraArgs, "--empty-ninja-file")
 	}
@@ -178,7 +178,7 @@
 	mainSoongBuildInvocation := primaryBuilderInvocation(
 		config,
 		soongBuildTag,
-		config.MainNinjaFile(),
+		config.SoongNinjaFile(),
 		mainSoongBuildExtraArgs)
 
 	if config.bazelBuildMode() == mixedBuild {
@@ -401,7 +401,7 @@
 
 	if config.SoongBuildInvocationNeeded() {
 		// This build generates <builddir>/build.ninja, which is used later by build/soong/ui/build/build.go#Build().
-		targets = append(targets, config.MainNinjaFile())
+		targets = append(targets, config.SoongNinjaFile())
 	}
 
 	ninja("bootstrap", ".bootstrap/build.ninja", targets...)