Remove the copy-paste that was folder[view]_fast_scan_tree, and

Remove the copy-paste that was folder[view]_fast_scan_tree, and
instead fix folder_scan_tree to do the right thing when called
in "Check for new folders" mode: Don't rescan the contents of
all folders after having updated the list of folders. There's
"Check for new messages" for that.
Fixes bug #2119, "Check for new folders on remote mailboxes
is very slow".