audio/zam-plugins-lv2: Drop nop and misspelled xLDFLAGS
Per [1] it already links with libzita-convolver on its own.
[1] http://beefy11.nyi.freebsd.org/data/head-i386-default/p502184_s348025/logs/zam-plugins-lv2-3.10.13_3.log