RISC-V: Remove N-extension related defines

The RISC-V N-extension is still in draft state hence remove
N-extension related defines from asm/csr.h.
Signed-off-by: NAnup Patel <anup.patel@wdc.com>
Signed-off-by: NPalmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/include/asm/csr.h

@@ -51,13 +51,10 @@
 #define CAUSE_IRQ_FLAG		(_AC(1, UL) << (__riscv_xlen - 1))
 
 /* Interrupt causes (minus the high bit) */
-#define IRQ_U_SOFT		0
 #define IRQ_S_SOFT		1
 #define IRQ_M_SOFT		3
-#define IRQ_U_TIMER		4
 #define IRQ_S_TIMER		5
 #define IRQ_M_TIMER		7
-#define IRQ_U_EXT		8
 #define IRQ_S_EXT		9
 #define IRQ_M_EXT		11