target/avr: Remove pointless use of CONFIG_USER_ONLY definition

Commit f1c671f ("target/avr: Introduce basic CPU class object")
added to target/avr/cpu.h:

  #ifdef CONFIG_USER_ONLY
  #error "AVR 8-bit does not support user mode"
  #endif

Remove the CONFIG_USER_ONLY definition introduced by mistake in
commit 7827168 ("cpu: tcg_ops: move to tcg-cpu-ops.h, keep a
pointer in CPUClass").

Reported-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-By: Warner Losh <imp@bsdimp.com>
Message-Id: <20210911165434.531552-2-f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

target/avr/cpu.c

@@ -197,10 +197,7 @@ static const struct TCGCPUOps avr_tcg_ops = {
     .synchronize_from_tb = avr_cpu_synchronize_from_tb,
     .cpu_exec_interrupt = avr_cpu_exec_interrupt,
     .tlb_fill = avr_cpu_tlb_fill,
-
-#ifndef CONFIG_USER_ONLY
     .do_interrupt = avr_cpu_do_interrupt,
-#endif /* !CONFIG_USER_ONLY */
 };
 
 static void avr_cpu_class_init(ObjectClass *oc, void *data)