box: drop the sequence field check from the _index.on_replace

Sequences can only be attached to integer fields of a space. The
corresponding check is done in both schema.lua and alter.cc. The
problem is that in order to make the index alter transactional,
we have to perform the `_index` space tuple replace before the
existed sequence is unattached from the space (because the index
alter may be yielding). And the check existing in the alter.cc
won't allow us to do that in some cases.

Since the check already exists in `box.schema.index.alter`, it's
decided to drop it from the `_index` `on_replace` trigger.

The downside of the solution is that uncareful manual altering the
index can silently put the space into an inconsistent condition:
the next insert into the space can either fail or silently succeed
whereas it's not supposed operation for the space. Example:

```lua
box.cfg{}
s = box.schema.space.create('s')
pk = s:create_index('pk', {parts = {{1, 'unsigned'}}, sequence = true})

-- Manual altering the index without dropping the sequence.
box.space._index:update({s.id, pk.id}, {{'=', 6, {{0, 'string'}}}})

-- This will fail: passed integer (from sequence) in string field.
s:insert({box.NULL})

-- Manual altering the index without dropping the sequence again.
box.space._index:update({s.id, pk.id}, {{'=', 6, {{0, 'scalar'}}}})

-- This will succeed: passed integer (from sequence) in scalar field.
-- Though it shouldn't have been done at all: one should not pass
-- NULL in the field which is indexed by the primary key.
s:insert({box.NULL})
```

NO_DOC=no visible changes
NO_TEST=no visible changes
NO_CHANGELOG=no visible changes