Remove unused BufferHubBase::GetBlobFds method

This method is not used in DVR buffering system. BufferHubBuffer may
consider having it later when it meets an use case when the buffer
contains multiple fds in buffer handle, and directly have it implemented
in BufferHubBuffer class.

Test: build
Bug: None
Change-Id: I95cd71d936695eeb77bd049f43fe350fb60d9154