Remove f-string from DAP

One more f-string snuck into the DAP code, in breakpoint.py.  Most of
them were removed here:

    https://sourceware.org/pipermail/gdb-patches/2023-June/200023.html

but I think this one landed after that patch.

While DAP only supports Python 3.5 and later, f-strings were added in
3.6, so remove this.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30708