transaction: make transaction_free() static

... as it's not needed outside transaction.c anymore.

src/core/transaction.c

@@ -34,7 +34,7 @@ static void transaction_delete_unit(Transaction *tr, Unit *u) {
                 transaction_delete_job(tr, j, true);
 }
 
-void transaction_abort(Transaction *tr) {
+static void transaction_abort(Transaction *tr) {
         Job *j;
 
         assert(tr);

src/core/transaction.h

@@ -34,4 +34,3 @@ int transaction_add_job_and_dependencies(
 int transaction_activate(Transaction *tr, Manager *m, JobMode mode, Set *affected, sd_bus_error *e);
 int transaction_add_isolate_jobs(Transaction *tr, Manager *m);
 int transaction_add_triggering_jobs(Transaction *tr, Unit *u);
-void transaction_abort(Transaction *tr);