Revert "Workaround for a change necessary for the Linux build."

This reverts commit f4be1866eb68e2889c96ac298e43217764ee61aa.
rpc.(cpp|h) renamed to bitcoinrpc.(cpp|h) to work around conflicting file names on Windows
https://github.com/phelixnmc/namecoin-qt/commit/f4be1866eb68e2889c96ac298e43217764ee61aa#commitcomment-4007654
(port from Bitcoin)