Remove remaining required references from Form\Schema.

The requiredness of a field comes from the field having a validation
rule.

src/Form/Schema.php

@@ -35,7 +35,6 @@ class Schema {
 		'type' => null,
 		'length' => null,
 		'precision' => null,
-		'required' => false,
 	];
 
 /**

tests/TestCase/Form/SchemaTest.php

@@ -51,7 +51,7 @@ class SchemaTest extends TestCase {
 
 		$this->assertEquals(['name'], $schema->fields());
 		$res = $schema->field('name');
-		$expected = ['type' => 'string', 'length' => null, 'required' => false];
+		$expected = ['type' => 'string', 'length' => null, 'precision' => null];
 		$this->assertEquals($expected, $res);
 
 		$res = $schema->addField('email', 'string');
@@ -59,7 +59,7 @@ class SchemaTest extends TestCase {
 
 		$this->assertEquals(['name', 'email'], $schema->fields());
 		$res = $schema->field('email');
-		$expected = ['type' => 'string', 'length' => null, 'required' => false];
+		$expected = ['type' => 'string', 'length' => null, 'precision' => null];
 		$this->assertEquals($expected, $res);
 	}
 
@@ -72,7 +72,7 @@ class SchemaTest extends TestCase {
 		$schema = new Schema();
 
 		$schema->addField('name', ['derp' => 'derp', 'type' => 'string']);
-		$expected = ['type' => 'string', 'length' => null, 'required' => false];
+		$expected = ['type' => 'string', 'length' => null, 'precision' => null];
 		$this->assertEquals($expected, $schema->field('name'));
 	}