rxrpc: Remove deadcode

rxrpc: Remove deadcode

Remove three functions that are no longer used.

rxrpc_get_txbuf() last use was removed by 2020's
commit 5e6ef4f1017c ("rxrpc: Make the I/O thread take over the call and
local processor work")

rxrpc_kernel_get_epoch() last use was removed by 2020's
commit 44746355ccb1 ("afs: Don't get epoch from a server because it may be
ambiguous")

rxrpc_kernel_set_max_life() last use was removed by 2023's
commit db099c625b13 ("rxrpc: Fix timeout of a call that hasn't yet been
granted a channel")

Both of the rxrpc_kernel_* functions were documented.  Remove that
documentation as well as the code.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Acked-by: David Howells <dhowells@redhat.com>
Link: https://patch.msgid.link/20250422235147.146460-1-linux@treblig.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>