tee: remove unnecessary NULL check in tee_shm_alloc()

Smatch complains that "ctx" isn't checked consistently:

    drivers/tee/tee_shm.c:164 tee_shm_alloc()
    warn: variable dereferenced before check 'ctx' (see line 95)

I audited the callers and "ctx" can't be NULL so the check can be
removed.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>

drivers/tee/tee_shm.c

@@ -161,8 +161,7 @@ struct tee_shm *tee_shm_alloc(struct tee_context *ctx, size_t size, u32 flags)
 		}
 	}
 
-	if (ctx)
-		teedev_ctx_get(ctx);
+	teedev_ctx_get(ctx);
 
 	return shm;
 err_rem: