WIP: Remove outdated benchmarking instructions.

[jdmoorman]

Resolves #689

This PR removes the instructions for manually running the pyperf benchmarks from README.rst. It also fixes the tox environment (tox -e perf) for running the benchmarks, which was missing a dependency on twisted.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@0b1f80f). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #690   +/-   ##
=========================================
  Coverage          ?   95.31%           
=========================================
  Files             ?       18           
  Lines             ?     2712           
  Branches          ?      321           
=========================================
  Hits              ?     2585           
  Misses            ?      104           
  Partials          ?       23           

Impacted Files	Coverage Δ
jsonschema/_format.py	79.31% <0.00%> (ø)
jsonschema/validators.py	94.71% <0.00%> (ø)
jsonschema/_types.py	100.00% <0.00%> (ø)
jsonschema/cli.py	96.96% <0.00%> (ø)
jsonschema/tests/test_format.py	94.23% <0.00%> (ø)
jsonschema/tests/_suite.py	96.29% <0.00%> (ø)
jsonschema/compat.py	58.97% <0.00%> (ø)
jsonschema/tests/test_validators.py	98.57% <0.00%> (ø)
jsonschema/__init__.py	100.00% <0.00%> (ø)
jsonschema/_utils.py	94.73% <0.00%> (ø)
... and 8 more

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[Julian]

Merging, thanks!