Removed unused GCC references

Remove Toolchain.GccRoot, Toolchain.GccVersion, and Toolchain.GccTriple
that were only used by an unused function.  This removes most of the
references to GCC, although there are still some left in the host
toolchains.

Test: No change to build.ninja for aosp_cf_x86_64_phone-userdebug or aosp_raven-userdebug
Change-Id: I72b8af1f9aa83e6e15c9e00ed1e817b6cc3a031a