jobspec-default: drop jobtap plugin

Problem: jobspec defaults will now be assigned by the job-frobnicator
rather than the jobspec-default jobtap plugin.

Drop the jobspec-default jobtap plugin.
Update sharness tests.

Author: garlick

src/modules/job-manager/Makefile.am

@@ -68,7 +68,6 @@ libjob_manager_la_SOURCES = \
 	jobtap.h \
 	jobtap.c \
 	plugins/priority-default.c \
-	plugins/jobspec-default.c \
 	plugins/limit-job-size.c \
 	plugins/limit-duration.c \
 	plugins/dependency-after.c \

src/modules/job-manager/jobtap.c

@@ -42,7 +42,6 @@
 #define FLUX_JOBTAP_PRIORITY_UNAVAIL INT64_C(-2)
 
 extern int priority_default_plugin_init (flux_plugin_t *p);
-extern int jobspec_default_plugin_init (flux_plugin_t *p);
 extern int limit_job_size_plugin_init (flux_plugin_t *p);
 extern int limit_duration_plugin_init (flux_plugin_t *p);
 extern int after_plugin_init (flux_plugin_t *p);
@@ -56,7 +55,6 @@ struct jobtap_builtin {
 
 static struct jobtap_builtin jobtap_builtins [] = {
     { ".priority-default", priority_default_plugin_init },
-    { ".jobspec-default", jobspec_default_plugin_init },
     { ".limit-job-size", limit_job_size_plugin_init },
     { ".limit-duration", limit_duration_plugin_init },
     { ".dependency-after", after_plugin_init },