wal: drop rows_per_wal option

Rows_per_wal option was deprecated because it can be covered by
wal_max_size. In order not to complicate WAL code with that
option's support this commit drops it completely.

In some tests the option was used to create several small xlog
files. Now the same is done via wal_max_size. Where it was
needed, number of rows per wal is estimated as wal_max_size / 50.
Because struct xrow_header size ~= 50 not counting paddings and
body.

Note, file box/configuration.result was deleted here, because it
is a stray result file, and it contained the rows_per_wal option
mentioning. Its test was dropped much earlier in
fdc3d1dd.

Closes #3762