build.gradle: remove 'prism.order' property for headless task.

For headless test task, 'prism.order' property is used to choose the
graph renderer to use. Currently, we specify this property to be 'sw'.

However, this property seems to trigger some unknown bug with
JDK11. This property will cause Java Runtime Error for Windows OS
including AppVoyer:

# A fatal error has been detected by the Java Runtime Environment:
#
#  EXCEPTION_ACCESS_VIOLATION (0xc0000005) at pc=0x00007ffd95b64879, pid=1476, tid=2640
#
# JRE version: OpenJDK Runtime Environment (11.0.1+13) (build 11.0.1+13)
# Java VM: OpenJDK 64-Bit Server VM (11.0.1+13, mixed mode, tiered, compressed oops, g1 gc, windows-amd64)
# Problematic frame:
# C  [javafx_font.dll+0x4879]

It is more like a JDK 11 bug with 'sw' renderer. Meanwhile, it is not so
necessary to set this property. It is acceptable to use platform-specified
graph renderer to do the test. Let’s remove this property to solve the
problem.

build.gradle

@@ -184,7 +184,6 @@ task headless {
             systemProperties = [
                 'testfx.robot': 'glass',
                 'testfx.headless': 'true',
-                'prism.order': 'sw',
                 'prism.text': 't2k',
             ]
         }