SUNRPC: Remove unused xdr functions

SUNRPC: Remove unused xdr functions

Remove a bunch of unused xdr_*decode* functions:
  The last use of xdr_decode_netobj() was removed in 2021 by:
commit 7cf96b6d0104 ("lockd: Update the NLMv4 SHARE arguments decoder to
use struct xdr_stream")
  The last use of xdr_decode_string_inplace() was removed in 2021 by:
commit 3049e974a7c7 ("lockd: Update the NLMv4 FREE_ALL arguments decoder
to use struct xdr_stream")
  The last use of xdr_stream_decode_opaque() was removed in 2024 by:
commit fed8a17c61ff ("xdrgen: typedefs should use the built-in string and
opaque functions")

  The functions xdr_stream_decode_string() and
xdr_stream_decode_opaque_dup() were both added in 2018 by the
commit 0e779aa70308 ("SUNRPC: Add helpers for decoding opaque and string
types")
but never used.

Remove them.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Link: https://lore.kernel.org/r/20250712233006.403226-1-linux@treblig.org
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>