FileAPI: Remove FileReader from Blob reading utilities

[KhafraDev]

Some environments that implement Blob do not implement FileReader, forcing them to skip these tests.

For example: https://github.com/nodejs/node/blob/main/test/wpt/status/FileAPI/blob.json#L7

[KhafraDev]

I took a look at as many uses of test_blob and test_blob_binary as I could find and there didn't seem to be anything special about the tests that isn't covered by the dedicated FileReader tests.

I could have missed something, but from what I remember of the FileReader tests, they adequately covered everything from here.