Removing gtar from base

[Tim Kientzle]

My plan with bsdtar has been to have both bsdtar and
gtar in 5.x, bsdtar only in 6.x.

I'd like to remove gtar from -CURRENT as the next
step in that transition.

Proposed timeline:
    * 1 week from today (March 19): Disable WITH_GTAR in -CURRENT.
    * End of March: Disconnect gtar from build in -CURRENT.
    * End of May: Remove gtar from -CURRENT.

Note:
    * gtar will remain in 5.x tree indefinitely.
    * WITH_GTAR will continue to be supported in 5.x.
    * gtar will continue to be available in ports indefinitely.

If there are no objections, I'll start this
process 1 week from today.

Tim Kientzle