PL2: Remove unused variable.

This doesn't fire until libc++ is updated. Presumably this used to
have some side effect that it shouldn't have had.

Test: m checkbuild
Bug: None
Change-Id: Ic60e0885799143efde23b91b05e5c1afd8356ae9

bootctrl/boot_control.cpp

@@ -525,7 +525,6 @@ int set_active_boot_slot(struct boot_control_module *module, unsigned slot)
 	int rc = -1;
 	int is_ufs = gpt_utils_is_ufs_device();
 	map<string, vector<string>>::iterator map_iter;
-	vector<string>::iterator string_iter;
 
 	if (boot_control_check_slot_sanity(module, slot)) {
 		ALOGE("%s: Bad arguments", __func__);