Remove unused seeder/compat.h

Summary:
This file is no longer used in the seeder since it is included in the
main tree (src/compat.h is used).

Test Plan:

./autogen.sh
mkdir build && cd build
../configure
make
./src/bitcoin-seeder

Ensure the software runs as expected

cd ..
mkdir buildcmake && cd buildcmake
cmake -GNinja ..
ninja
./src/bitcoin-seeder

Ensure the software runs as expected

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2728