Clean up useless mention of RMGRDESCSOURCES in pg_rewind Makefile.

RMGRDESCSOURCES is defined and used only in pg_xlogdump Makefile,
but pg_rewind Makefile mentioned it as extra files to remove in "make clean".
This patch removes that useless mention from pg_rewind Makefile.

Michael Paquier

src/bin/pg_rewind/Makefile

@@ -24,7 +24,7 @@ OBJS	= pg_rewind.o parsexlog.o xlogreader.o datapagemap.o timeline.o \
 	fetch.o file_ops.o copy_fetch.o libpq_fetch.o filemap.o logging.o \
 	$(WIN32RES)
 
-EXTRA_CLEAN = $(RMGRDESCSOURCES) xlogreader.c
+EXTRA_CLEAN = xlogreader.c
 
 all: pg_rewind