remove some unused functions

Summary:
Identified with cppcheck --enable=unusedFunction ..

• GetSendBufferSize()'s last use removed in 991955ee81034dc3fbc1c2a8e60c04fc9e0b538c
• SetPort()'s last use removed in 7e195e8459ad741368db6bb574981fccb1707268
• GetfLargeWorkInvalidChainFound() was introduced in e3ba0ef95636290a3bb597ddd25d13ea13b034aa and never used

This is a backport of core PR10501

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1887