Hide host filter mode if there's only one

[kidonng]

Makes Ctrl R cycling through modes more convenient, as host in this case is the same as global.

[kidonng]

🎉 Since 18.4.0 filter modes are now configurable, thanks to #2430:

[search]
## The list of enabled filter modes, in order of priority.
## The "workspace" mode is skipped when not in a workspace or workspaces = false.
## Default filter mode can be overridden with the filter_mode setting.
# filters = [ "global", "host", "session", "workspace", "directory" ]