Remove misleading line from dx.jar Android.mk file.

Previously we had a line in the dx.jar Android.mk file that indicates
that dx.jar is compiled with java 1.6. This was a lie and it has been
being compiled with java 1.8 for quite a while.

Bug: 28464522
Change-Id: Idc21295d7aca6afced16c46c16951a4aa66535d0