Bump the build time requirement, and assumed runtime, version of
shared-mime-info to 1.3, which seems to be a good low-enough baseline
according to:
https://repology.org/metapackage/shared-mime-info/versions
This makes it possible to drop a number of definitions provided by
newer versions of shared-mime-info:
- application/metalink+xml
- application/x-mimearchive
- application/x-mobipocket-ebook
- application/x-nzb
- application/x-trig
- audio/aac
- image/webp
Also, drop text/x-objchdr too, as it has no magic nor glob, hence it is
basically useless.