flux-job: rename kill/raise source file

Problem: 'flux job cancel' was removed but job/cancel.c still
exists, containing the 'kill' and 'raise' subcommands.

Rename the source file to job/kill.c/

Author: garlick

src/cmd/Makefile.am

@@ -160,7 +160,7 @@ flux_job_SOURCES = \
 	job/id.c \
 	job/namespace.c \
 	job/urgency.c \
-	job/cancel.c \
+	job/kill.c \
 	job/eventlog.c \
 	job/info.c \
 	job/stats.c \