Remove unused GetType() from CBufferedFile and CAutoFile

Summary:
GetType() is only called in tests, so it is unused and can be removed.

While touching all constructors, the class name can be adjusted to comply with the style guide.

This concludes backport of core#28458
https://github.com/bitcoin/bitcoin/pull/28458/commits/fa2f2413b87f5fc1e5c92bf510beebdcd0091714
https://github.com/bitcoin/bitcoin/pull/28458/commits/fa19c914f7fe7be127c0fb330b41ff7c091f40aa
Depends on D19145

Test Plan: ninja all check-all bitcoin-fuzzers

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19147