Remove libdvm support from the core build system.

Now libart is the only supported runtime and
we don't need the build variables PRODUCT_RUNTIMES and
DALVIK_VM_LIB.

Bug: 18465297
Change-Id: Ibfda931cde0649163d79b584fb5ccad927a9bc2b
Merge "Remove libdvm support from the core build system."