Remove outdated TODO comment in lib/std/start.zig

[magurotuna]

Hi, I'm new to zig and I'm so fascinated with it that I just started looking for some quick issues to tackle as the first contribution for me.

• Finish taking advantage of sentinel-terminated pointers in the standard library #3767

I thought this issue should be a good start, soon realizing that apparently it is already resolved except that one outdated TODO comment remains there. In fact, grep gives me the following result:

❯ rg 'issues/265'
lib/std/start.zig
364:// TODO https://github.com/ziglang/zig/issues/265

And the function that follows this TODO comment, posixCallMainAndExit, seems to be already taking advantage of sentinel-terminated pointers (I suppose this commit is relevant). So I have concluded that this comment should be outdated.

Resolves #3767

[jedisct1]

Hi Yusuke!

And thank you so much for your first contribution 🥰

Yep, looks like this comment is indeed not relevant any more. Good catch!