[Docs] Remove SVN checkout from LLDB build steps

This removes several older paragraphs in the LLDB build steps for Unix
systems which suggested checking out various components via SVN. Since
there's a separate page about getting the LLDB source which only
mentions Git, it seems appropriate to remove this older info from the
build docs.

Patch by: J. Ryan Stinnett

Differential revision: https://reviews.llvm.org/D62041

llvm-svn: 361046