Drop minor GetSerializeSize template

Summary:
Now that GetType() is not propagated, the benefits are not worth the code.

This is a partial backport of Core PR13558 : https://github.com/bitcoin/bitcoin/pull/13558/commits/893628be0166b4096b6e52f516e0f65bb63a75a2

Depends on D4605 and D4603

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4606