[ruby/rubygems] Remove `verify_gz`

This PR removes the `#verify_gz` method because it is redunant and
unnecessary.

Previously the `data.tar.gz` would get read twice for every file - once
in `verify_gz` and once in `extract_files`. The `extract_files` method
verifies the `data.tar.gz` when it reads it, and raises an error if
unzipping it fails.

The `verify_gz` code can be seen in some profiles as a hotspot -
although not major - as it accounts for between 9% and 17% of time, but
only when the installation thread doesn't have native extensions or
plugins.

https://github.com/ruby/rubygems/commit/737c82986c

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