Revert "Flag to control if generated SDK prebuilts are preferred."

This reverts commit d0b99cea0ac389e831dfddbd3cb10fa09a040d2c.

Reason for revert: Not needed anymore. We will simple exclude the 
gernated Android.bp files from prebuilt drops instead, it keeps
things simpler.

Change-Id: I5b01b2e01197f9fceb4630c8ade95f0d1d0f33f4
diff --git a/sdk/update.go b/sdk/update.go
index 2ab45d7..b146b62 100644
--- a/sdk/update.go
+++ b/sdk/update.go
@@ -32,9 +32,8 @@
 // ========================================================
 //
 // SOONG_SDK_SNAPSHOT_PREFER
-//     By default every unversioned module in the generated snapshot has prefer set by the
-//     sdk.prebuilts_prefer property. Building it with SOONG_SDK_SNAPSHOT_PREFER=true will force
-//     them to use prefer: true.
+//     By default every unversioned module in the generated snapshot has prefer: false. Building it
+//     with SOONG_SDK_SNAPSHOT_PREFER=true will force them to use prefer: true.
 //
 // SOONG_SDK_SNAPSHOT_VERSION
 //     This provides control over the version of the generated snapshot.
@@ -1624,11 +1623,11 @@
 
 	// Do not add the prefer property if the member snapshot module is a source module type.
 	if !memberType.UsesSourceModuleTypeInSnapshot() {
-		// Set the prefer based on the environment variable if present, else the sdk.prefer_prebuilts
-		// value.
+		// Set the prefer based on the environment variable. This is a temporary work around to allow a
+		// snapshot to be created that sets prefer: true.
 		// TODO(b/174997203): Remove once the ability to select the modules to prefer can be done
 		//  dynamically at build time not at snapshot generation time.
-		prefer := ctx.sdkMemberContext.Config().IsEnvTrue("SOONG_SDK_SNAPSHOT_PREFER") || s.PreferPrebuilts()
+		prefer := ctx.sdkMemberContext.Config().IsEnvTrue("SOONG_SDK_SNAPSHOT_PREFER")
 
 		// Set prefer. Setting this to false is not strictly required as that is the default but it does
 		// provide a convenient hook to post-process the generated Android.bp file, e.g. in tests to