Remove AdHoc provider, as part of #3515 channel removal (#3676)

# Changed Behaviour

Functionality removal for anyone using the AdHocProvider

## Type of change

- New feature
- Code maintenance/cleanup

Author: benclifford

parsl/channels/base.py

@@ -17,8 +17,8 @@ class Channel(metaclass=ABCMeta):
     such as push_file, pull_file and makedirs.
 
     Channels should ensure that each launched command runs in a new process
-    group, so that providers (such as AdHocProvider and LocalProvider) which
-    terminate long running commands using process groups can do so.
+    group, so that providers (such as LocalProvider) which terminate long
+    running commands using process groups can do so.
     """
 
     @abstractmethod

parsl/providers/ad_hoc/__init__.py

@@ -135,13 +135,6 @@
     #     # For example:
     #     'remote_writeable': 'globus://af7bda53-6d04-11e5-ba46-22000b92c6ec/home/bzc/'
     # },
-    # 'adhoc': {
-    #    # This specifies configuration parameters when testing an ad-hoc SSH based cluster
-    #    'username': 'fixme', # username on remote systems
-    #    'remote_hostnames': ['hostname1', 'hostname2'], # addresses of remote systems
-    #    'worker_init': 'init commands',  # worker_init for remote systems
-    #    'script_dir': "/path"  # script directory on remote systems
-    # }
     #
 }  # type: Dict[str, Any]