default.xml: remotes, TC comment, remove unneeded GCC

1) Remotes: add remotes for BitBucket and ThankYouLuigi (AOSPA Legacy blobs)

2) TC comment: warn team members of delta resolution freeze when syncing latest SaberMod toolchains on our DrDevs server and how to properly mitigate the issue.

3) Remove unneeded GCC: we are not building for MIPS or x86, so we do not need these toolchains.