Fix bigserial appears with limit 8 for schema dumper

[kamipo]

Before:

create_table "postgresql_big_serials", force: :cascade do |t|
  t.bigserial "seq", limit: 8, null: false
end

After:

create_table "postgresql_big_serials", force: :cascade do |t|
  t.bigserial "seq", null: false
end

[rails-bot]

r? @arthurnn

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[kamipo]

Currently schema dumper dumps bigserial with limit: 8.
But it is not intended behavior because bigserial(8) is invalid statement.

activerecord_unittest=# create table t (a bigserial(8));
ERROR:  type modifier is not allowed for type "bigint"
LINE 1: create table t (a bigserial(8));

Actually limit: 8 behaves nothing for bigserial because NATIVE_DATABASE_TYPES doesn't include :bigserial.
lib/active_record/connection_adapters/postgresql_adapter.rb#L74-L113
But I think it is better to do not dump limit: 8 for bigserial.