Remove unreachable fallback code for tray_output primary.

Remove unreachable fallback code for tray_output primary.

This commit removes the code for falling back to the first available
output for the system tray if 'tray_output primary' has been specified
but there is no primary output (managed by this bar).

This fallback behavior was broken/unreachable because the tray
will never be initialized in this situation in the first place. Having
this dead code lead to a wrong assumption in #1855 and hence to
commit e2e7b70d002cac2031cb65d6f5f197c9583913d6, which makes the
system tray not show up for many users when first installing i3.

Thanks to @rtlanceroad for reporting this issue.

fixes #2220