shared/logs-show: Remove unused OUTPUT_FOLLOW

shared/logs-show: Remove unused OUTPUT_FOLLOW

As of the commit aae9a96d4b3a8562af9e8c6a23871b442645b954 removing --follow
option in systemctl command, OUTPUT_FOLLOW has never been set anywhere. Let's
remove it.

The condition expression of the if-statement in show_journal() that refers to
OUTPUT_FOLLOW now thus evaluates always to true. Hence, the call of
sd_journal_wait() is in dead code, and the outer infinite for-loop is
meaningless, which we remove as cleanup.

There is no functional change by this commit.