[test] Remove self-test for empty formatted setting value

Commit 5ad445f ("[settings] Treat an empty formatted value as meaning
"delete setting"") (re)defined the semantics of storing an empty
formatted setting as meaning "delete setting".

Remove the existing self-test using an empty formatted hex setting
value, since it no longer conforms to the defined semantics.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/tests/settings_test.c

 		    RAW ( 0xf2, 0x37, 0xb2, 0x18 ), "0xf237b218" );
 
 	/* "hex" setting type */
-	storef_ok ( &test_settings, &test_hex_setting,
-		    "", RAW ( 0x00 ) );
 	storef_ok ( &test_settings, &test_hex_setting,
 		    ":", RAW ( 0x00, 0x00 ) );
 	storef_ok ( &test_settings, &test_hex_setting,