Standardize synonym type definition for PLURAL

We're working towards standardizing synonym type definitions across the OBO Foundry. This involved getting OMO terms for several common synonym type definitions. More information in OBOFoundry/OBOFoundry.github.io#2450.

In this PR, the ad hoc definition in ZFA for PLURAL synonyms is replaced with the standard synonym type definition OMO:0003004 for plural syonyms