usrp.pc.in: Drop unset LIBUSB_PKG_CONFIG_NAME

That variable is expected to be set by macro USRP_LIBUSB
from m4/usrp_libusb.m4, but it is never called in configure.ac.
Furthermore, the macro doesn't seem to be working fine in some
environments like debian9, while it works on others (ArchLinux), so
let's better drop it.

Change-Id: Ia5f7ec69701abff91ff8e903e0fd53f8d9fbfba6

usrp.pc.in

@@ -5,7 +5,7 @@ includedir=@includedir@
 
 Name: usrp
 Description: USRP Client Side C++ interface
-Requires: @LIBUSB_PKG_CONFIG_NAME@
+Requires: libusb-1.0
 Version: @VERSION@
 Libs: -L${libdir} -lusrp
 Cflags: -I${includedir}