Remove doc reference to "import -t" (for now)

It is currently broke due to 'set -u' and on testing, I can't seem
to get it working properly based on the docs/help:

$ sudo singularity import -t docker ubuntu /tmp/Moo.img
ERROR: No Docker image specified (with --file)

So to work around the problem and not delay the release, I am hiding
the feature so we can still debug and fix, but not delay release on
a broken feature.

libexec/cli/import.help

@@ -22,7 +22,6 @@ note: This command must be executed as root.
 IMPORT OPTIONS:
     -f/--file       Use an input file instead of a pipe for tar, or
                     name an image of the specified format to import
-    -t/--type       Source type ('tar' or 'docker', default: tar)
        --command    Replace the tar command (DEFAULT: 'tar xvf -')
 
 EXAMPLES:
@@ -39,11 +38,6 @@ EXAMPLES:
     $ sudo singularity import -f Debian.tar /tmp/Debian.img
     $ docker export [container] | sudo singularity import [container].img
 
-    Converting a Docker Fedora image:
-
-    $ sudo singularity import -t docker fedora /tmp/fedora.img
-
-
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