tools: stop repeating lines to delete for multiple bfd timer changes

tools: stop repeating lines to delete for multiple bfd timer changes

Problem reported with tracebacks seen when making multiple bfd timer
changes in frr.conf and applying via frr-reload.py.  Found that when
multiple bfd timer changes are made, the same line can be added for
deletion more than once, causing the traceback when the deletion is
performed.  This fix verifies the correct line is being appended for
deletion.

Ticket: CM-27233
Signed-off-by: Don Slice <dslice@cumulusnetworks.com>