tests: kernel: interrupt: Do not test interrupt nesting on RISC-V

This commit disables the nested interrupt test for the RISC-V platform,
as interrupt nesting is not supported on the current RISV-C
architecture port.

Furthermore, the current `trigger_irq` implementation for RISC-V is
mostly incorrect and cannot be used, so there is no point in leaving
that in the codebase (see zephyrproject-rtos#23593).

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

tests/kernel/interrupt/src/interrupt_util.h

@@ -60,17 +60,6 @@ static inline void trigger_irq(int irq)
 #endif
 }
 
-#elif defined(CONFIG_RISCV)
-static inline void trigger_irq(int irq)
-{
-	u32_t mip;
-
-	printk("Triggering irq : %d\n", irq);
-	__asm__ volatile ("csrrs %0, mip, %1\n"
-			: "=r" (mip)
-			: "r" (1 << irq));
-}
-
 #elif defined(CONFIG_CPU_ARCV2)
 static inline void trigger_irq(int irq)
 {