target/avr: Remove pointless use of CONFIG_USER_ONLY definition

target/avr: Remove pointless use of CONFIG_USER_ONLY definition

Commit f1c671f96cb ("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 78271684719 ("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>