Remove the EXPERIMENTAL_USE_OPENJDK11_TOOLCHAIN option.

Previously, this setting environment variable to false reverted to an
OpenJDK 9 toolchain. This change removes that option.

Test: m core-all-system-modules
Test: zcat out/verbose.log.gz | grep 'prebuilts/jdk/jdk[0-9]*'
Test: EXPERIMENTAL_USE_OPENJDK11_TOOLCHAIN=false m core-all-system-modules (fails)
Bug: 131683177
Change-Id: I0d679648e1236925ce3ef7e1652379127e846b00