gdb/dwarf: remove unnecessary comparison in cooked_index_entry::compare

gdb/dwarf: remove unnecessary comparison in cooked_index_entry::compare

I believe that the `(mode == MATCH && a == munge ('<'))` part of the
condition is unnecesary.  Or perhaps I don't understand the algorithm.

The use of "munge" above effectively makes it so that the template
portion of names is completely ignored for the sake of the comparison.

Then, in the condition, this:

    a == munge ('<')

is functionally equivalent to

    a == '\0'

If `a` is indeed '\0', and `b` is also '\0', then we would have taken
the earlier branch:

    if (a == b)
      return 0;

If `b` is not '\0', then we won't take this branch and we'll go into the
final comparison:

    return a < b ? -1 : 1;

So, as far as I can see, there is no case where `mode == MATCH`, where
we're going to use this special `return 0`.

Regression tested using the various DWARF target boards on Debian 12.

Change-Id: I5ea0463c1fdbbc1b003de2f0a423fd0073cc9dec
Approved-By: Tom Tromey <tom@tromey.com>