RFR: 8323242: Remove vestigial DONT_USE_REGISTER_DEFINES

[Koichi Sakata]

This pull request removes an unnecessary directive.

There is no definition of DONT_USE_REGISTER_DEFINES in HotSpot or the build system, so this `#ifndef`conditional directive is always true. We can remove it. 

I built OpenJDK with Zero VM as a test. It was successful.


$ ./configure --with-jvm-variants=zero --enable-debug
$ make images 
$ ./build/macosx-aarch64-zero-fastdebug/jdk/bin/java -version                                                                                                                                         
openjdk version "23-internal" 2024-09-17                                                                                                                                                                                                                                                 
OpenJDK Runtime Environment (fastdebug build 23-internal-adhoc.jyukutyo.jyukutyo-jdk)                                                                                                                                                                                                    
OpenJDK 64-Bit Zero VM (fastdebug build 23-internal-adhoc.jyukutyo.jyukutyo-jdk, interpreted mode)


It may be possible to remove the `#define noreg` as well because the CONSTANT_REGISTER_DECLARATION macro creates a variable named noreg, but I can't be sure. When I tried removing the noreg definition and building the OpenJDK, the build was successful.

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Commit messages:
 - Remove DONT_USE_REGISTER_DEFINES

Changes: https://git.openjdk.org/jdk/pull/18115/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18115&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323242
  Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18115.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18115/head:pull/18115

PR: https://git.openjdk.org/jdk/pull/18115