Remove the virtual specifier for functions with the override specifier

Summary:
override guarantees that the function is virtual (in addition
to that the function is overriding a virtual function from a base
class).

This is backport of Core PR10701

Test Plan:

make check

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3118