Remove libLLVM* from art

The functionality in libbacktrace that depended on libLLVM* has been
split out into libbacktrace_offline.  Remove the unnecessary
dependencies from dex2oats.

Test: m -j dex2oats ART_BUILD_HOST_STATIC=true
Change-Id: I79b99c4d420caefbdf6aa81541120ddedd2149c5