Remove gradle-wrapper arg

It is the default behavior to use the project's Gradle wrapper if available.

plugin/src/compatTest/kotlin/org/nixos/gradle2nix/WrapperTest.kt

@@ -0,0 +1,22 @@
+package org.nixos.gradle2nix
+
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.io.TempDir
+import java.io.File
+import kotlin.test.assertEquals
+
+class WrapperTest {
+    @TempDir
+    lateinit var root: File
+
+    @Test
+    fun `resolves gradle version from wrapper configuration`() {
+        val model = root.buildKotlin("""
+            tasks.wrapper {
+                gradleVersion = "5.5.1"
+            }
+        """.trimIndent())
+
+        assertEquals(model.gradle.version, "5.5.1")
+    }
+}