[cmake] Remove unused ${LLDB_PLUGINS} dependency from our Objective-C++ CMake config

Summary:
LLDB_PLUGINS doesn't exist as a variable, so this line doesn't add any dependencies and is
just confusing. It seems this slipped in from the gdb-remote CMake I was using as a CMake template.

The gdb-remote CMake itself is using a local LLDB_PLUGINS variable, so that code is fine.

Reviewers: davide

Reviewed By: davide

Subscribers: davide, mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D49695

llvm-svn: 337741