configure.ac: Remove final traces of avahi (#558)

configure.ac: Remove final traces of avahi (#558)

This was originally searched for cups-browsed, and during the great
split these lookups accidentally got left in and moved around in the
file.

The original code began with this autoconf comment:
```
dnl Avahi for cups-browsed
```

After shifting down, it became:
```
dnl Avahi
```

After removing avahi from configure.ac, there are no remaining mentions
of avahi anywhere other than CHANGES*.md