Remove icu4j dependency.

This dependency is no longer required: it was added
for libcore when libcore was implemented in terms of
com.icu classes. libcore has since moved to android.icu
(in core-libart) and so the droiddoc dependency is no
longer required.

Test: make droid docs
Change-Id: I6e4e79a7df201fdc2cefa60037adf831593eaa0f