Remove SSH channels, as part of #3515 channel removal (#3677)

# Changed Behaviour

This removes functionality for all users of the SSH channel
implementations. There are more details on alternatives in issue #3515.

## Type of change

- New feature
- Code maintenance/cleanup

Author: benclifford

parsl/channels/base.py

@@ -8,12 +8,9 @@ class Channel(metaclass=ABCMeta):
 
     For certain resources such as campus clusters or supercomputers at
     research laboratories, resource requirements may require authentication.
-    For instance some resources may allow access to their job schedulers from
-    only their login-nodes which require you to authenticate through SSH, or
-    require two factor authentication.
 
-    The simplest Channel, *LocalChannel*, executes commands locally in a
-    shell, while the *SSHChannel* authenticates you to remote systems.
+    The only remaining Channel, *LocalChannel*, executes commands locally in a
+    shell.
 
     Channels provide the ability to execute commands remotely, using the
     execute_wait method, and manipulate the remote file system using methods