Drop obsolete javac -extdirs commandline arg.

The extension directory defaults to lib/ext and does not
exist by default. Setting it to the empty string de facto
disables this obsolete feature.

AOSP is moving to a hermetic toolchain so this argument
will stop working soon. Further, OpenJDK 9 javac no longer
supports this command line argument when compiling for
-source 1.9 -target 1.9.

This command line argument has been around since the
earliest versions of Android, but is now obsolete.
This CL drops it.

Bug: 63746471
Test: Treehugger
Change-Id: Ia0214c1b192e3ffda10772d777557a81ce346c03