ixpmanager.conf: Dedupe dbase_host and dbase_hostname (latter is used)

Having grepped the entire tree and even submodules, I can't find anything that references the `dbase_host` entry in `tools/perl-lib/IXPManager/ixpmanager.conf`. Everything seems to reference the `dbase_hostname` entry. It seems the first is a redundant entry. This removes it.