disable coverage checking

I don't think we ever use this, and it slows things down. If we want to use it,
we should just do so on a couple of builds rather than all of them.

tox.ini

@@ -3,7 +3,6 @@ envlist = packaging, py27, py36, pep8, check_isort
 
 [base]
 deps =
-    coverage
     Twisted>=17.1
     mock
     python-subunit
@@ -26,9 +25,7 @@ passenv = *
 
 commands =
     /usr/bin/find "{toxinidir}" -name '*.pyc' -delete
-    coverage run {env:COVERAGE_OPTS:} --source="{toxinidir}/synapse" \
     "{envbindir}/trial" {env:TRIAL_FLAGS:} {posargs:tests} {env:TOXSUFFIX:}
-    {env:DUMP_COVERAGE_COMMAND:coverage report -m}
 
 [testenv:py27]