spec: cleanup

- remove unnecessary explicit library requires
- reword summary
- change package group to "System Environment/Libraries"
- simplify libtool archive removal command
- compression-independent manpage path

Based on feedback from Michael Schwendt:
https://bugzilla.redhat.com/show_bug.cgi?id=920751#c2

Signed-off-by: Martin Milata <mmilata@redhat.com>