Removes unneeded await from `bot.is_owner()` function

Summary

Fixes #239.

[x] If code changes were made then they have been tested.
```
import discord
from discord.ext import commands
```

bot = commands.Bot(command_prefix="test!", self_bot=False) bot.owner_ids = [876055467678375998]

@bot.event async def on_ready(): print("logged in")

@bot.command() @commands.is_owner() async def ping(ctx): await ctx.reply(f"Pong, {round(bot.latency, 1)}ms.", mention_author=False)

bot.run()


- [x] This PR fixes an issue (please put issue # in summary).