RFR: 8242456: PreviewFeature.Feature enum removal of TEXT_BLOCKS

[Jan Lahoda]

The enum constant is unused, and can be removed. The class is not an API.

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Commit messages:
 - 8242456: PreviewFeature.Feature enum removal of TEXT_BLOCKS

Changes: https://git.openjdk.java.net/jdk/pull/2218/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2218&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8242456
  Stats: 7 lines in 1 file changed: 0 ins; 7 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2218.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2218/head:pull/2218

PR: https://git.openjdk.java.net/jdk/pull/2218