This reverts commit 3efc6433.

- better to toggle this case by case in regression tests / build environments
  using the "spack spec FOO ^libfabric@develop" notation

- suggested by Todd Gamblin.  There are two depends_on() statements now.
  The second one will only be triggered on cray platform, and will be merged
  with prevous depends_on

- this is specifically for the benefit of GNI provider; Mercury master depends
  on provider functionality that isn't in a tagged libfabric release yet
- this change should really be conditional on platform.  Querying spack
  developers for help on how to do this.