security/clamav: really do the right thing with the MSPACK option.

previous patch added the configure argument to find archivers/libmspack
installed, but neglected to set USES+= pkgconfig to actually have the binary
use that version. With it now properly finding libmspack.so, revert to
not trying to install libclammspack* if the MSPACK option is set.

PR: 233658