Remove legacy dead code in DDP script launch

[awaelchli]

What does this PR do?

Part of #11643

Found while reviewing #11643.
The removed code snippet refers to DDPStrategy.num_processes being None, but this is never True since we set that value at the time of Trainer instantiation. This is legacy dead code and can be safely removed.

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Did you have fun?

I made sure I had fun coding 🙃

Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

cc @Borda @justusschock @awaelchli @akihironitta @rohitgr7 @kaushikb11

[codecov]

Codecov Report

Merging #11678 (9c04996) into master (7da931d) will increase coverage by 36%.
The diff coverage is 100%.

❗ Current head 9c04996 differs from pull request most recent head 7ac8660. Consider uploading reports for the commit 7ac8660 to get more accurate results

@@            Coverage Diff            @@
##           master   #11678     +/-   ##
=========================================
+ Coverage      56%      92%    +36%     
=========================================
  Files         197      195      -2     
  Lines       17224    17124    -100     
=========================================
+ Hits         9619    15734   +6115     
+ Misses       7605     1390   -6215