README: Remove all references to --with-verbs[*]

Now that all use of libibverbs is gone from Open MPI, and all
verbs-based configury is also removed, update README to remove all
references to --with-verbs.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

README

@@ -1193,51 +1193,6 @@ NETWORKING SUPPORT / OPTIONS
 --with-usnic
   Abort configure if Cisco usNIC support cannot be built.
 
---with-verbs=<directory>
-  Specify the directory where the verbs (also known as OpenFabrics
-  verbs, or Linux verbs, and previously known as OpenIB) libraries and
-  header files are located.  This option is generally only necessary
-  if the verbs headers and libraries are not in default
-  compiler/linker search paths.
-
-  The Verbs library usually implies operating system bypass networks,
-  such as InfiniBand, usNIC, iWARP, and RoCE (aka "IBoIP").
-
---with-verbs-libdir=<directory>
-  Look in directory for the verbs libraries.  By default, Open MPI
-  will look in <verbs_directory>/lib and <verbs_ directory>/lib64,
-  which covers most cases.  This option is only needed for special
-  configurations.
-
---with-verbs-usnic
-  Note that this option is no longer necessary in recent Linux distro
-  versions.  If your Linux distro uses the "rdma-core" package (instead
-  of a standalone "libibverbs" package), not only do you not need this
-  option, you shouldn't use it, either.  More below.
-
-  This option will activate support in Open MPI for disabling a
-  dire-sounding warning message from libibverbs that Cisco usNIC
-  devices are not supported (because Cisco usNIC devices are supported
-  through libfabric, not libibverbs).  This libibverbs warning can
-  also be suppressed by installing the "no op" libusnic_verbs plugin
-  for libibverbs (see https://github.com/cisco/libusnic_verbs, or
-  download binaries from cisco.com).
-
-  This option is disabled by default for two reasons:
-
-  1. It causes libopen-pal.so to depend on libibverbs.so, which is
-     undesirable to many downstream packagers.
-  2. As mentioned above, recent versions of the libibverbs library
-     (included in the "rdma-core" package) do not have the bug that
-     will emit dire-sounding warnings about usnic devices.  Indeed,
-     the --with-verbs-usnic option will enable code in Open MPI that
-     is actually incompatible with rdma-core (i.e., cause Open MPI to
-     fail to compile).
-
-   If you enable --with-verbs-usnic and your system uses the rdma-core
-   package, configure will safely abort with a helpful message telling
-   you that you should not use --with-verbs-usnic.
-
 
 RUN-TIME SYSTEM SUPPORT