Revert the move of the god check to the top of isMaster in db/replutil.h

There is no clear reason why this was done in the first place and the code which would seemingly be the cause has been changed in a way to no longer even hint at needing this behavior.

Commits:
https://github.com/mongodb/mongo/commit/6446345ba4836ba1cd70b6c30296f96df260ba5e
https://github.com/mongodb/mongo/commit/bcc17bed5e07053f6d5f946c840fc25bc9bafb2b#L12L115