Revert "ANDROID: GKI: net: add vendor hooks for 'struct nf_conn' lifecycle"

This reverts commit 4d309564.

The hooks android_rvh_nf_conn_alloc and android_rvh_nf_conn_free were
not used by any vendor, so remove them to help with merge issues with
future LTS releases.

If this is needed by any real user, it can easily be reverted to add it
back and then the symbol should be added to the abi list at the same
time to prevent it from being removed again later.

Bug: 203756332
Bug: 171013716
Cc: Vignesh Saravanaperumal <vignesh1.s@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I069a798036056feadb2c933da42d405b7bf22cd4