riscv: Remove unused GENERATING_ASM_OFFSETS

The macro is not used in the current version of kernel, it looks like
can be removed to avoid a build warning:

../arch/riscv/kernel/asm-offsets.c: At top level:
../arch/riscv/kernel/asm-offsets.c:7: warning: macro "GENERATING_ASM_OFFSETS" is not used [-Wunused-macros]
    7 | #define GENERATING_ASM_OFFSETS

Fixes: 9639a44 ("RISC-V: Provide a cleaner raw_smp_processor_id()")
Cc: stable@vger.kernel.org
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Chunyan Zhang <zhangchunyan@iscas.ac.cn>
Link: https://lore.kernel.org/r/20241008094141.549248-2-zhangchunyan@iscas.ac.cn
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: Chunyan Zhang

arch/riscv/kernel/asm-offsets.c

@@ -4,8 +4,6 @@
  * Copyright (C) 2017 SiFive
  */
 
-#define GENERATING_ASM_OFFSETS
-
 #include <linux/kbuild.h>
 #include <linux/mm.h>
 #include <linux/sched.h>