Remove forced -p (progress) flag from phpcs.xml

This causes phpcs to emit invalid JSON when the --report=json
flag is set and no issues are found, which breaks editor plugins
(atom-linter-phpcs in my case).

Add this flag in composer.json instead.

Change-Id: Ibb0b4c084e25e06be72389f1d4998804cfd53295

composer.json

@@ -6,7 +6,7 @@
 	"scripts": {
 		"test": [
 			"parallel-lint . --exclude vendor",
-			"phpcs"
+			"phpcs -p"
 		]
 	}
 }

phpcs.xml

@@ -7,6 +7,5 @@
 	<arg name="encoding" value="utf8"/>
 	<arg name="extensions" value="php,php5,inc"/>
 	<arg name="colors"/>
-	<arg value="p"/>
 	<exclude-pattern>vendor</exclude-pattern>
 </ruleset>