Remove sysctl_vfs_cache_pressure assumption

The generic SPL cache shrinkers make the assumption that the
caches only contain VFS cache data and therefore should be scaled
based on vfs_cache_pressure. This is not strictly true and it
should not be assumed.

Removing this tuning should not have any impact on the stock
behavior because vfs_cache_pressure=100 by default. This means
that no scaling will take place.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>