init: remove last init.cpp global

By moving it into builtins.cpp..., but that's less bad than it is
now, especially since this is defunct in code targeting Q+.  Remove
the guards that init.h isn't being included by other files too as it's
not useful anymore.

Test: build
Change-Id: Ic564fcff9e8716ec924098b07a8c9d94ca25f960
diff --git a/init/builtins.cpp b/init/builtins.cpp
index f188ef9..fca9809 100644
--- a/init/builtins.cpp
+++ b/init/builtins.cpp
@@ -89,6 +89,8 @@
 namespace android {
 namespace init {
 
+std::vector<std::string> late_import_paths;
+
 static constexpr std::chrono::nanoseconds kCommandRetryTimeout = 5s;
 
 static Result<void> reboot_into_recovery(const std::vector<std::string>& options) {