Clean files associated with static linking
auto merge of #1941 : larsbergstrom/servo/clean_rlib, r=metajack

Three things changed with the `make clean` process:
- Remove the *.rlib files associated with the Servo crates
- Remove the libservo-*.a file, which we will (Very Soon Now) generate now on Android instead of a .so
- Update all the submodules, which have corresponding *.rlib file cleanup changes