cpu: Replace ENV_GET_CPU with env_cpu

Now that we have both ArchCPU and CPUArchState, we can define
this generically instead of via macro in each target's cpu.h.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

target/m68k/cpu.h

@@ -168,8 +168,6 @@ static inline M68kCPU *m68k_env_get_cpu(CPUM68KState *env)
     return container_of(env, M68kCPU, env);
 }
 
-#define ENV_GET_CPU(e) CPU(m68k_env_get_cpu(e))
-
 #define ENV_OFFSET offsetof(M68kCPU, env)
 
 void m68k_cpu_do_interrupt(CPUState *cpu);