Fix ARI warning in thread-thread.c::dec_thread_get_ada_task_ptid

Fix ARI warning in thread-thread.c::dec_thread_get_ada_task_ptid

This patch fixes the following ARI warning:

    gdb/dec-thread.c:695: regression: multi-line string: Multi-line string
    must have the newline escaped

I think the new-line was unintentional, so I simply removed it,
and then reformatted the string to fit within our 70-80 max characters-
per-line rule.

gdb/ChangeLog:

       * dec-thread.c (dec_thread_get_ada_task_ptid): Avoid unescaped
       new-line in debug string.  Remove trailing spaces.