Remove unneeded libmd from bootstrap-tools (reverting r246784).

The bootstrap-tools are supposed to be host tools, which in most cases, use
host headers and libraries. As such, directly including the src tree's headers
for libmd here causes the need to link libmd in since it will be built with
the new symbols (which /usr/lib/libmd.so) won't have unless it is new enough.
During the target build in buildworld the target headers are staged into
WORLDTMP and used via --sysroot, allowing the target xinstall to be built with
the new/target libmd.

The .PATH here was also not doing anything since xinstall does not use libmd
source files.

Sponsored by: EMC / Isilon Storage Division
MFC after: 2 weeks