comms/uhd: Remove PORTDOCS macro

The PORTDOCS macro was duplicating the files already listed in
pkg-plist.

> Building package for uhd-4.6.0.0_1

pkg-static: duplicate file listing: /usr/local/share/doc/uhd/LICENSE, fatal (developer mode)
pkg-static: duplicate file listing: /usr/local/share/doc/uhd/README.md, fatal (developer mode)

Approved by: portmgr (blanket)