Removed code from ImapStore.internalOpen() that tried to get the path...

Removed code from ImapStore.internalOpen() that tried to get the path delimeter before SELECTing a mailbox. This shouldn't be a problem since the delimeter
a) is already known if the server supports the NAMESPACE capability.
b) is included in the prefix anyway (=also known if namespace was set manually).
c) isn't needed when selecting a mailbox because we currently don't support folder hierarchy. So the delimeter is included in the folder name.

Fixes issue 1217