firmware: Remove obsolete Chelsio cxgb3 firmware

The current firmware version used by the device driver
is 7.12.0

Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Ben Hutchings <ben@decadent.org.uk>
Cc: James Bottomley <JBottomley@Parallels.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Divy Le Ray <divy@chelsio.com>
Cc: netdev@vger.kernel.org
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>