Remove extra blank line between thread-root placeholder and replies

The placeholder row for an omitted thread root was followed by an
extra blank line, making the overflow view layout feel wobbly against
the normal rendered-parent case. Drop the trailing blank so the first
reply follows the placeholder directly.

Author: fprochazka

src/slackcli/output.py

@@ -240,7 +240,6 @@ def output_thread_text(
         replies = list(messages)
         placeholder_user = format_user_name(parent.user_name, parent.user_id)
         print(f"{parent.datetime_str}  {placeholder_user} [thread root message omitted -- use --head to see it]")
-        print()
     elif messages:
         parent = messages[0]
         replies = list(messages[1:])