riscv: drop unneeded -Wall addition

The top level Makefile adds -Wall globally:

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

For riscv, I see two "-Wall" added for compiling each object.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

arch/riscv/Makefile

@@ -34,8 +34,6 @@ else
 	KBUILD_LDFLAGS += -melf32lriscv
 endif
 
-KBUILD_CFLAGS += -Wall
-
 # ISA string setting
 riscv-march-$(CONFIG_ARCH_RV32I)	:= rv32ima
 riscv-march-$(CONFIG_ARCH_RV64I)	:= rv64ima