cleanup(pubsub): remove handwritten create subscription from samples.cc

[alevenberg]

#12987

Exists in subscription_admin_samples.cc now

Also, moved the tag in google3 already.

This change is 

[snippet-bot]

Here is the summary of changes.

You are about to delete 1 region tag.

• google/cloud/pubsub/samples/samples.cc:126, tag pubsub_create_pull_subscription

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[codecov]

Codecov Report

Attention: Patch coverage is 64.28571% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 93.30%. Comparing base (f0af9a3) to head (c218952).

Files	Patch %	Lines
google/cloud/pubsub/samples/samples.cc	64.28%	15 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13676   +/-   ##
=======================================
  Coverage   93.30%   93.30%           
=======================================
  Files        2228     2228           
  Lines      193359   193366    +7     
=======================================
+ Hits       180404   180412    +8     
+ Misses      12955    12954    -1     

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