Remove linker selection from hb-ucdn

Since this is a convenience library, it's not actually linked. Or so I think.
Hoping this might fix this Appveyor failure:

https://ci.appveyor.com/project/harfbuzz/harfbuzz/build/job/9884jyb5vryja9pw

Making all in hb-ucdn
make[4]: Entering directory '/c/projects/harfbuzz/src/hb-ucdn'
  CC       libhb_ucdn_la-ucdn.lo
  GEN      libhb-ucdn.la
/bin/sh: libhb_ucdn_la-ucdn.lo: command not found
make[4]: *** [Makefile:404: libhb-ucdn.la] Error 127
make[4]: Leaving directory '/c/projects/harfbuzz/src/hb-ucdn'
make[3]: *** [Makefile:2548: all-recursive] Error 1
make[3]: Leaving directory '/c/projects/harfbuzz/src'
make[2]: *** [Makefile:1382: all] Error 2
make[2]: Leaving directory '/c/projects/harfbuzz/src'
make[1]: *** [Makefile:486: all-recursive] Error 1
make[1]: Leaving directory '/c/projects/harfbuzz'
make: *** [Makefile:418: all] Error 2

src/hb-ucdn/Makefile.am

@@ -4,17 +4,6 @@ noinst_LTLIBRARIES = libhb-ucdn.la
 
 include Makefile.sources
 
-if OS_WIN32
-libhb_ucdn_la_LINK = $(CXXLINK)
-else
-# Use a C linker for GCC, not C++; Don't link to libstdc++
-if HAVE_GCC
-libhb_ucdn_la_LINK = $(LINK)
-else
-libhb_ucdn_la_LINK = $(CXXLINK)
-endif
-endif
-
 libhb_ucdn_la_SOURCES = $(LIBHB_UCDN_sources)
 libhb_ucdn_la_CPPFLAGS = \
 	-I$(top_srcdir) \