Remove default libraries

libdl is part of system_shared_libs now. -ldl -lpthread -lm are now defaults
for host_ldlibs on Linux and Darwin. -lrt is a default for host_ldlibs on
Linux.

Test: m host
Change-Id: I0b3c147b00a8ab6ff289b85db55b88836c905f5c
Exempt-From-Owner-Approval: build system cleanup