Remove explicit server from the check-keys.sh script

Summary:
There is no reason to enforce using this server rather than another.
Relying on the user configuration seems to be a better solution, as the
key servers tend to be all synchronized.

Test Plan:

./check-keys.sh

Check the keys are retrieved or already known

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2420