fix(cros_setup_toolchains): Do not install setup cross gdb.

We've never had a need for cross-gdb builds and my new toolchain builder
script doesn't bother building one. As such this old toolchain installer
script should stop trying to install it. :)
Merge pull request #14 from marineam/no-gdb

fix(cros_setup_toolchains): Do not install setup cross gdb.