Fix memory leak

Command owns parameters.

11126== 24 bytes in 1 blocks are possibly lost in loss record 819 of 4,774

11126== at 0x4C29216: operator new(unsigned long) (vg_replace_malloc.c:334)

11126== by 0x4628D5BA: Command::processParameter(QStringList const&) (Command.cpp:258)

11126== by 0x46283CDA: KisGmicParser::createFilterTree() (kis_gmic_parser.cpp:204)

11126== by 0x462816EB: KisGmicPlugin::slotShowGmicDialog() (kis_gmic_plugin.cpp:208)