imp: remove kill subcommand

Problem: The `flux-imp kill` subcommand is no longer necessary,
since both IMP exec and run subcommands now have a privleged IMP
parent process via which signals may be forwarded.

Remove the `kill` subcommand and associated tests.

src/imp/Makefile.am

@@ -62,7 +62,6 @@ IMP_SOURCES = \
 	signals.h \
 	cgroup.c \
 	cgroup.h \
-	kill.c \
 	run.c \
 	exec/user.h \
 	exec/user.c \

src/imp/impcmd-list.c

@@ -18,8 +18,6 @@ extern int imp_casign_unprivileged (struct imp_state *imp, struct kv *);
 extern int imp_casign_privileged (struct imp_state *imp, struct kv *);
 extern int imp_exec_unprivileged (struct imp_state *imp, struct kv *);
 extern int imp_exec_privileged (struct imp_state *imp, struct kv *);
-extern int imp_kill_unprivileged (struct imp_state *imp, struct kv *);
-extern int imp_kill_privileged (struct imp_state *imp, struct kv *);
 extern int imp_run_unprivileged (struct imp_state *imp, struct kv *);
 extern int imp_run_privileged (struct imp_state *imp, struct kv *);
 
@@ -41,9 +39,6 @@ struct impcmd impcmd_list[] = {
     { "exec",
       imp_exec_unprivileged,
       imp_exec_privileged },
-    { "kill",
-      imp_kill_unprivileged,
-      imp_kill_privileged },
     { "run",
       imp_run_unprivileged,
       imp_run_privileged },

t/Makefile.am

@@ -21,7 +21,6 @@ TESTSCRIPTS = \
 	t1002-sign-munge.t \
 	t1003-sign-curve.t \
 	t2000-imp-exec.t \
-	t2001-imp-kill.t \
 	t2002-imp-run.t \
 	t2003-imp-exec-pam.t