test: trim field name update from serializer test

The test re encoding of -2^63 Lua number value did use update by a field
name, which does not supported in 1.10 and 2.2 branches. Field name
updates are orthogonal to Lua number serialization and we don't intend
to test them here. So it is safe and logical to get rid of them in the
test.

This change allow the test to pass on 1.10 and 2.2 branches.

Follows up #4672.

Reviewed-by: Alexander Tikhonov <avtikhon@tarantool.org>
Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>