Remove the Amount::GetSatoshi function

Summary:
Also always include ostream as the class is using std::ostream . The iostream on OSX over includes and not including it on other plateform rely on transitive inclusion, which is not desirable.

Depends on D1669, D1670 and D1671

Test Plan:

make check

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1672