Comment out default auto-loading of `spec/support`

Continuing with the effort to help RSpec suites run faster in isolation;
disable the previously default autoloading of `spec/support` files. With
larger suites, this autoloading had the tendency to load more extraneous
configurations than necessary.

The suggested alternative is to manually require any necessary support
files in the `*_spec.rb` files. Since RSpec has already added `spec/` to
the load path, this is a simple require:

    require 'support/some_file'

This has the pleasant (depending on your point of view) side-effect or
making one think critically about how support files are defined.

Close #1106