BuildSystem: Avoid an extra case for OpenBSD.

BuildSystem: Avoid an extra case for OpenBSD.

Nine years ago with Mantis 13639 (now ASTERISK-12841) an extra case for OpenBSD
was introduced: Vorbis required Ogg to be specified manually, because the shared
library libvorbis.so did not specify its required dependency on -logg itself.

Today with OpenBSD 6.2, all libvorbis*.so declare their dependencies correctly.
Therefore, an extra case is not required anymore.

Change-Id: Ifd04e0994ce9f1e4ad29c3948a0398b91d1e97bc