Remove unnecessary switch to groff

GUI/xephem/Makefile

@@ -28,14 +28,11 @@ ifndef MOTIFL
 MOTIFL = /usr/lib
 endif
 
-ROFF = nroff
-
 # For Linux and Apple OS X
 # /opt/homebrew is for modern Apple Silicon; the other paths, for older macOS.
 
 UNAME_S = $(shell uname -s)
 ifeq ($(UNAME_S),Darwin)
-  ROFF = nroff
   PLATI = -I/opt/homebrew/include \
           -I/usr/local/opt/openssl@3/include \
           -I/opt/X11/include
@@ -206,7 +203,7 @@ xephem: $(INCS) $(OBJS)
 	$(CC) $(LDFLAGS) -o $@ $(OBJS) $(LIBS)
 
 xephem.1: xephem.man
-	$(ROFF) -man $? > $@
+	nroff -man $? > $@
 
 libs:
 	cd ../../libastro; make