cleanup(pubsub): remove handwritten subscription api samples

[alevenberg]

#12987

Note:

• Samples exists in subscription_admin_samples.cc now
• I moved the tag in google3 already.
• Some of the delete's are not ordered anymore (1,2,3...). They will eventually all be gone so this does not matter.

This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.25%. Comparing base (16319a7) to head (0507136).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13682      +/-   ##
==========================================
- Coverage   93.26%   93.25%   -0.01%     
==========================================
  Files        2229     2229              
  Lines      193299   193199     -100     
==========================================
- Hits       180272   180175      -97     
+ Misses      13027    13024       -3     

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[snippet-bot]

You are about to delete the following sample browser pages.

• google/cloud/pubsub/samples/samples.cc:153, tag pubsub_create_subscription_with_filter(usage: page 1).
• google/cloud/pubsub/samples/samples.cc:179, tag pubsub_create_push_subscription(usage: page 1).
• google/cloud/pubsub/samples/samples.cc:205, tag pubsub_create_unwrapped_push_subscription(usage: page 1).
• google/cloud/pubsub/samples/samples.cc:232, tag pubsub_create_cloud_storage_subscription(usage: page 1).
• google/cloud/pubsub/samples/samples.cc:417, tag pubsub_update_push_configuration(usage: page 1).

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Here is the summary of changes.

You are about to delete 5 region tags.

• google/cloud/pubsub/samples/samples.cc:153, tag pubsub_create_subscription_with_filter
• google/cloud/pubsub/samples/samples.cc:179, tag pubsub_create_push_subscription
• google/cloud/pubsub/samples/samples.cc:205, tag pubsub_create_unwrapped_push_subscription
• google/cloud/pubsub/samples/samples.cc:232, tag pubsub_create_cloud_storage_subscription
• google/cloud/pubsub/samples/samples.cc:417, tag pubsub_update_push_configuration

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[alevenberg]

This PR is ready for review