docs/manual: ditch redundant -f 'rm' flag

The value of the RM variable in make is 'rm -f' [0], thus the additional
-f is redundant. Avoid it on the docs to avoid developers taking it as a
good example to follow.

[0] https://www.gnu.org/software/make/manual/make.html#index-RM

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

docs/manual/writing-rules.txt

@@ -78,7 +78,7 @@ Do not align the +=+ signs.
 +
 ---------------------
 define LIBFOO_REMOVE_DOC
-	$(RM) -fr $(TARGET_DIR)/usr/share/libfoo/doc \
+	$(RM) -r $(TARGET_DIR)/usr/share/libfoo/doc \
 		$(TARGET_DIR)/usr/share/man/man3/libfoo*
 endef
 ---------------------
@@ -118,7 +118,7 @@ YES:
 ---------------------
 ifneq ($(BR2_LIBFOO_INSTALL_DATA),y)
 define LIBFOO_REMOVE_DATA
-	$(RM) -fr $(TARGET_DIR)/usr/share/libfoo/data
+	$(RM) -r $(TARGET_DIR)/usr/share/libfoo/data
 endef
 LIBFOO_POST_INSTALL_TARGET_HOOKS += LIBFOO_REMOVE_DATA
 endif
@@ -128,7 +128,7 @@ NO:
 +
 ---------------------
 define LIBFOO_REMOVE_DATA
-	$(RM) -fr $(TARGET_DIR)/usr/share/libfoo/data
+	$(RM) -r $(TARGET_DIR)/usr/share/libfoo/data
 endef
 
 ifneq ($(BR2_LIBFOO_INSTALL_DATA),y)