IB: Remove reference to obsolete CONFIG_IPATH_CORE

Since commit b1c1b6a3 ("IB/ipath: merge ipath_core and ib_ipath
drivers"), CONFIG_IPATH_CORE no longer exists, so there's no reason to
have a line for it in drivers/Makefile.

Pointed out by Robert P. J. Day <rpjday@mindspring.com>.

Signed-off-by: Roland Dreier <rolandd@cisco.com>