Remove platform plugins from lldb-server

Summary:
This removes the last usage of Platform plugins in lldb-server -- it was used for launching child
processes, where it can be trivially replaced by Host::LaunchProces (as lldb-server is always
running on the host).

Removing platform plugins enables us to remove a lot of other unused code, which was pulled in as
a transitive dependency, and it reduces lldb-server size by 4%--9% (depending on build type and
architecture).

Reviewers: clayborg

Subscribers: tberghammer, danalbert, srhines, lldb-commits

Differential Revision: http://reviews.llvm.org/D20440

llvm-svn: 274125