RFR: JDK-8299554: Remove EnableIf from metaprogramming/enableIf.hpp

[Justin King]

As the title says, remove `EnableIf` type alias from `metaprogramming/enableIf.hpp` leaving only `ENABLE_IF` and `ENABLE_IF_SDEFN`.

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Commit messages:
 - Remove EnableIf from metaprogramming/enableIf.hpp

Changes: https://git.openjdk.org/jdk/pull/11835/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11835&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299554
  Stats: 168 lines in 7 files changed: 6 ins; 5 del; 157 mod
  Patch: https://git.openjdk.org/jdk/pull/11835.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11835/head:pull/11835

PR: https://git.openjdk.org/jdk/pull/11835