Remove pkg/bootstrap from our go distribution

This accounts for 46MB out of 291MB of our final packages, and isn't
included in the binary packages downloaded from golang.org.

The source files inside bootstrap also include comments with absolute
paths from the builder, which isn't nice.

Test: prebuilts/build-tools/build-prebuilts.sh, compare dist/go.zip
Test: m -j blueprint_tools on linux with this change
Change-Id: I901590efa9726fcecfc94da51616fb8c89ac8b8c