Remove unused template

Previously, code does not compile if using Post(fence) method. Error
message is no matching member function for call to 'Post',
frameworks/native/libs/vr/libbufferhub/include/private/dvr/buffer_hub_client.h:217:7:
note: candidate template ignored: couldn't infer template argument
'Meta'.

Test: use Post method in test and build on oc-dr1-daydream-dev branch.
Bug: None
Change-Id: Icc6977ec6b805d062cb3d0289c73fbf8b43ec940