Remove superfluous coverage erase.

`coverage run` starts clean unless `--append` is specified.

Author: aaugustin

tox.ini

@@ -16,7 +16,6 @@ pass_env = WEBSOCKETS_*
 
 [testenv:coverage]
 commands =
-    python -m coverage erase
     python -m coverage run --source {envsitepackagesdir}/websockets,tests -m unittest {posargs}
     python -m coverage report --show-missing --fail-under=100
 deps = coverage