NFSD: Remove unused nfsd4_compoundargs::cachetype field

[ Upstream commit 77e378cf ]

This field was added by commit 1091006c ("nfsd: turn on reply
cache for NFSv4") but was never put to use.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>