job-manager: drop unused #define

Problem: job-manager defines DEFAULT_JOURNAL_SIZE_LIMIT but
this is no longer used.

Drop definition.

Author: garlick

src/modules/job-manager/journal.c

@@ -29,8 +29,6 @@
 #include "job.h"
 #include "journal.h"
 
-#define DEFAULT_JOURNAL_SIZE_LIMIT 1000
-
 struct journal {
     struct job_manager *ctx;
     flux_msg_handler_t **handlers;