staging: erofs: drop unneeded -Wall addition

The top level Makefile adds -Wall globally:

  KBUILD_CFLAGS   := -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs \

I see two "-Wall" added for compiling objects in drivers/staging/erofs/.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/erofs/Makefile

@@ -2,7 +2,7 @@
 
 EROFS_VERSION = "1.0pre1"
 
-ccflags-y += -Wall -DEROFS_VERSION=\"$(EROFS_VERSION)\"
+ccflags-y += -DEROFS_VERSION=\"$(EROFS_VERSION)\"
 
 obj-$(CONFIG_EROFS_FS) += erofs.o
 # staging requirement: to be self-contained in its own directory