Remove some unnecessary headers from Host.cpp

After the recent migration of code out of Host.cpp, many
unnecessary headers were still being included.  This prunes the
include list down to only what is still necessary.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@219814 91177308-0d34-0410-b5e6-96231b3b80d8