Merge #10568: Remove unnecessary forward class declarations in header files

Summary:
4087d9ea7 Remove unnecessary forward class declarations in header files (practicalswift)

Tree-SHA512: 80a9b2cd339ba9c606c0dd93739ac80941b6050388fee2edd7976ae8b5e332fd0b4f1df1e89c1d6d28a77c23a36b1e874e72977985fb8f710c65c45c3a926f97

Backport of Core PR10568
https://github.com/bitcoin/bitcoin/pull/10568/

Test Plan:

make check

Reviewers: jasonbcox, deadalnix, Fabien, O1 Bitcoin ABC, #bitcoin_abc, markblundeberg

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3261