Security v1-v2 transition has been completed for a while and path
separation is no longer needed. However, this commit preserves
`ndn::security::v2` namespace, but makes it inline.
Some commonly used v2 headers are preserved, but are deprecated and
should be avoided in the dependent software.
Change-Id: I26056cb3d34bcdc0a16e06dea8c1a78c03da2d40