devel/qt6-base: Drop unused textproc/xmlstarlet

xmlstarlet is only used as a last resort attempt to reduce MIME database
size if no actual file compression methods are found. Since we enable
zstd support, this is the effective compression method used.

PR: 285738
Reported by: diizzy
MFH: 2025Q2

(cherry picked from commit 7ac865e740c85737a68dbca87734efa4545d065b)