graphics/qgis-ltr: Avoid CMake warning MAKE_GRASS

CMake Warning:

Ignoring extra path from command line:
 "/usr/ports/graphics/qgis/work/.build/WITH_GRASS8"

This happens, because of unneeded CMAKE_OFF=WITH_GRASSx.
Not functional change, not bump of PORTREVISION necessary.

PR: 274793
Reported by: Martin Birgmeier <d8zNeCFG@aon.at>
MFH: 2024Q1