Remove unused GetType() from OverrideStream, CVectorWriter, SpanReader

Summary:
GetType() is never called, so it is completely unused and can be removed.

This is a partial backport of core#28508
https://github.com/bitcoin/bitcoin/pull/28508/commits/fa4a9c0f4334678fb80358ead667807bf2a0a153
Depends on D19142

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19143