Don't plumb PRODUCT_*_CLASSPATH vars to make.

They were used to generate export values for BOOTCLASSPATH and friends;
which are now generated by derive_classpath service based on proto
configs at runtime.

Bug: 180105615
Test: m && launch_cvd; presubmit / DeviceBootTest
Change-Id: Ifbf3f076a00e084d47a48180a4286fc53e610c5d