Removed supports_binary_field flag as all backends support them

It was mainly for MySQL on Python 3, but now the current
recommended MySQL driver for Python 3 (mysqlclient) does support
binary fields, it is unneeded. Refs #20377.

tests/migrations/test_operations.py

@@ -7,7 +7,6 @@ try:
 except ImportError:
     sqlparse = None
 
-from django import test
 from django.db import connection, migrations, models, transaction
 from django.db.migrations.migration import Migration
 from django.db.migrations.state import ProjectState
@@ -669,7 +668,6 @@ class OperationTests(OperationTestBase):
         self.assertEqual(pony.digits, "42")
         self.assertEqual(pony.quotes, '"\'"')
 
-    @test.skipUnlessDBFeature('supports_binary_field')
     def test_add_binaryfield(self):
         """
         Tests the AddField operation on TextField/BinaryField.