If a node reaches the flood stage watermark then we automatically apply
the `read_only_allow_delete` block to all its indices to prevent any
further growth in data. Users are expected to fix the disk space issue
by adding more space or deleting indices. However some users may prefer
to fix the disk space issues by modifying some of the index settings,
perhaps removing replicas or adjusting an allocation filter to move
shards onto nodes with more space. Today this isn't possible since the
`read_only_allow_delete` block also applies to metadata writes. Blocking
metadata writes isn't necessary to protect against further increases in
disk usage, and makes it harder for users to resolve the disk space
issue, so this commit removes the `METADATA_WRITE` level from the block
definition.
b1db7b67c6