RFR: 8289770: Drop IS_WINVISTA macro from ShellFolder2.cpp

[Alexey Ivanov]

This clean-up PR removes unused Windows version macro from `ShellFolder2.cpp`.

`IS_WINVISTA` was not used at all.

`IS_WINXP` guarded support for icons with alpha channel. It is now safe to assume Java runs on a Windows version later than Windows XP. Java launchers specify 6.0 as the minimum OS version which corresponds to Windows Vista.

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Commit messages:
 - 8289770: Drop IS_WINVISTA macro from ShellFolder2.cpp

Changes: https://git.openjdk.org/jdk/pull/18736/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18736&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289770
  Stats: 18 lines in 1 file changed: 0 ins; 13 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/18736.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18736/head:pull/18736

PR: https://git.openjdk.org/jdk/pull/18736