<AWT Dev> RFR: 8240290 Clean the "libawt_xawt" library from code for macOS and headless mode

[Sergey Bylokhov]

Hello.
Please review the fix for jdk/client.

Bug: https://bugs.openjdk.java.net/browse/JDK-8240290
Fix: http://cr.openjdk.java.net/~serb/8240290/webrev.00

The libawt_xawt.so library is used by the headful toolkit only and only on Unix,
but it still uses some code related to macOS(the time when macport project used XToolkit)
and headless mode, which should be deleted.

In the fix, all usage of #HEADLESS and #MACOSX in libawt_xawt were removed.
In each file the assert "#error This file " is added to prove that these
files are unused in headless mode.


mach5 is green

-- 
Best regards, Sergey.