Remove USE_BAZEL and its dead code

This environment variable has been out of use for some time. It used to
control Bazel-as-executor, which has been removed from Bazel for several
releases.

Bug: 243077098
Test: Treehugger
Change-Id: I4edfb5f3574c369a16547ea06ba780293c37ecb3

cmd/soong_ui/main.go

@@ -176,10 +176,6 @@ func main() {
 
 	build.SetupOutDir(buildCtx, config)
 
-	if config.UseBazel() && config.Dist() {
-		defer populateExternalDistDir(buildCtx, config)
-	}
-
 	// Set up files to be outputted in the log directory.
 	logsDir := config.LogsDir()