finance/alkimia: Remove WEBKIT option.

www/qt5-webkit was removed from the ports tree, so remove it as an
option and leave only WEBENGINE if the user wants webpage rendering
support.

Reported by: Andrea Venturoli <ml@netfence.it> (via private mail)
MFH: 2024Q1

(cherry picked from commit e352d3c164629e1fcc34a2072378c4bda56fa62e)