RFR: 8292244: Remove unnecessary include directories

[Daniel Jeliński]

Remove `java.base:include` from `EXTRA_HEADER_DIRS`.

The directory contains base versions of the files present in `<build>/support/modules_include/java.base`; the latter is automatically [included in every JDK library build](https://github.com/openjdk/jdk/blob/master/make/autoconf/flags-cflags.m4#L434).

Verified that:
- cl.exe command line contains `-I<stuff>/modules_include/java.base`
- Windows, Linux and Mac builds still pass

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Commit messages:
 - Remove one more
 - Simplify includes

Changes: https://git.openjdk.org/jdk/pull/9835/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9835&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292244
  Stats: 6 lines in 2 files changed: 0 ins; 5 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9835.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9835/head:pull/9835

PR: https://git.openjdk.org/jdk/pull/9835