Remove unnecessary dependencies on libsigchain.

surfaceflinger was required to link against libsigchain to dlopen
libart.so for a debugging option that was removed in change
Ib9a129329f7bd8d67b954e58810807c683b20b48. gpuservice accidentally
linked against it, presumably due to copy/paste from the surfaceflinger
Android.bp file.

Bug: http://b/120782499
Bug: http://b/135284876
Test: treehugger
Change-Id: I593c5483c8c096ac95f310c68f53000e06621d8b
Merged-In: I593c5483c8c096ac95f310c68f53000e06621d8b