multimedia/qmmp-qt5: fix leftover files

Remove INSTALL_ICONS to solve this build issue:

[00:11:58] =>> Checking for extra files and directories
[00:12:00] =>> Error: Files or directories left over:
[00:12:00] share/icons/hicolor/icon-theme.cache

INSTALL_ICONS is documented as not required for Qt-based ports.

PR: 254245
Submitted by: mandree@
Approved by: maintainer timeout, 16d, liangtai.s16@gmail.com