<AWT Dev> RFR: 8242559: Clean the "libawt_xawt" library from code for headless mode

[Sergey Bylokhov]

This bug extracted from JDK-8240290[1]

The libawt_xawt.so library is used by the headful toolkit, but it still
has some code related to the headless mode, which could be deleted.

In the fix, the HEADLESS related code was removed, to prove that we did
not build this library in HEADLESS mode, I added "#error This " to all
library files(but some of them already has it)

[1] https://bugs.openjdk.java.net/browse/JDK-8240290

The old review request:
https://mail.openjdk.java.net/pipermail/awt-dev/2020-April/015881.html

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Commit messages:
 - 8242559: Clean the "libawt_xawt" library from code for headless mode

Changes: https://git.openjdk.java.net/jdk/pull/101/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=101&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8242559
  Stats: 316 lines in 29 files changed: 119 ins; 167 del; 30 mod
  Patch: https://git.openjdk.java.net/jdk/pull/101.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/101/head:pull/101

PR: https://git.openjdk.java.net/jdk/pull/101