gdb: remove breakpoint_re_set_one

During a later patch I wanted to reset a single breakpoint, so I
called breakpoint_re_set_one.  However, this is not the right thing to
do.  If we look at breakpoint_re_set then we see that there's a whole
bunch of state that needs to be preserved prior to calling
breakpoint_re_set_one, and after calling breakpoint_re_set_one we
still need to call update_global_location_list.

I could just update the comment on breakpoint_re_set_one to make it
clearer how the function should be used -- or more likely to warn that
the function should only be used as a helper from breakpoint_re_set.

However, breakpoint_re_set_one is only 3 lines long.  So I figure it
might actually be easier to just fold breakpoint_re_set_one into
breakpoint_re_set, then there's no risk of accidentally calling
breakpoint_re_set_one when we shouldn't.

There should be no user visible changes after this commit.