Don't use storage as a class property [#3614921]

Problem/Motivation

Phpstan is saying, regarding a number of different classes, "Storing entity storage as a class property is not recommended. Call Drupal\Core\Entity\EntityTypeManagerInterface::getStorage() at the call-site instead." It also says "Direct injection of entity storage via $storage is not recommended. Inject Drupal\Core\Entity\EntityTypeManagerInterface and call getStorage() at the call-site instead." about a similarly long list of classes.

Proposed resolution

Evaluate whether the storage is actually being used; if not, remove it; if so, substitute the recommended methods.

Remaining tasks

Make a MR. I am assigning this to myself since I intend to work on it right away.

Issue fork feeds-3614921

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3614921-dont-use-storage changes, plain diff MR !255
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