[lldb-server] No need to add pthread twice.

Summary:
Following on from r247991:
pthread is in LLDB_SYSTEM_LIBS so there's no need to explicitly add it to the link.

Reviewers: labath

Subscribers: lldb-commits, labath, krytarowski

Differential Revision: http://reviews.llvm.org/D12964

llvm-svn: 248177