LU-18753 ptlrpc: remove unused stub functions

LU-18753 ptlrpc: remove unused stub functions

Remove:

ptlrpc_ping_import_soon()
flavor_copy()
ptlrpc_cleanup_client()
__lustre_swab_buf()

They are not called anywhere.

Fixes: 86b2211e55dc ("LU-290 Reconnects are not throttled")
Fixes: 3565394baa95 ("LU-3289 gss: Add userspace support for GSS null and sk")
Fixes: 3ee0e0908f12 ("LU-5829 ptlrpc: remove unnecessary EXPORT_SYMBOL")
Fixes: 23fad25a5b6b ("b=18631")
Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: Ib45aba0b76d086b0a657bb3fc79d1ec74b1e3302
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58226
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Aurelien Degremont <adegremont@nvidia.com>
Reviewed-by: Max Wang <wamax@amazon.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>