docs: Eliminate some unsatisfied references

[wcohen]

When attempting to build the documentation locally I noticed there were the following unsatisfied references with a make epub:

writing output... [100%] refs
/home/wcohen/research/profiling/pcp/pcp/docs/HowTos/scaling/index.rst:147: WARNING: undefined label: 'c. centralized logging (pmlogger farm)' [ref.ref]
/home/wcohen/research/profiling/pcp/pcp/docs/HowTos/scaling/index.rst:187: WARNING: undefined label: 'd. federated setup (multiple pmlogger farms)' [ref.ref]
/home/wcohen/research/profiling/pcp/pcp/docs/HowTos/scaling/index.rst:187: WARNING: undefined label: 'c. centralized logging (pmlogger farm)' [ref.ref]
/home/wcohen/research/profiling/pcp/pcp/docs/QG/QuickGuides.rst:46: WARNING: undefined label: 'establishing secure client connections' [ref.ref]
generating indices... genindex done

That patch update corrects the references in the scaling.rst HowTos and removes one that no longer has a matching entry in QuickGuides.rst.