Remove chan_usbradio and app_rpt.

These modules are being maintained outside of the tree and have been for a long
time now, so it doesn't make sense to keep them here.

Review: https://reviewboard.asterisk.org/r/1764/
........

Merged revisions 359050 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 359051 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@359052 65c4cc65-6c06-0410-ace0-fbb531ad65f3