task/install: don't attempt to handle strings for extra_system_packages

To the best of my knowledge, extra_system_packages has never accepted
a string as a value -- whether at the top level (i.e. on the install
task itself) or on a particular project (e.g. ceph).

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

Author: idryomov

teuthology/task/install/__init__.py

@@ -430,8 +430,6 @@ def _override_extra_system_packages(config, project, install_overrides):
 
         if isinstance(extra_overrides, list):
             extra_overrides = dict(deb=extra_overrides, rpm=extra_overrides)
-        elif isinstance(extra, str):
-            e = dict(deb=[extra], rpm=[extra])
 
         config['extra_system_packages'] = teuthology.deep_merge(e, extra_overrides)