Fix dev-lang/mono-9999 build (src_prepare())

Mono upstream no longer has mono/metadata/Makefile.am named mono/metadata/Makefile.am.in.  Trying to duplicate Makefile.am.in to Makefile.am is therefore pointless.  Removing this line allows src_prepare to succeed.  Change occurred in upstream at commit ( mono/mono@ad8236ebb857acf1459d7c87b85df82918dc2313 ).