RFR: JDK-8299554: Remove EnableIf from metaprogramming/enableIf.hpp [v2]

[Justin King]

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

Justin King has updated the pull request incrementally with one additional commit since the last revision:

  s/td/std
  
  Signed-off-by: Justin King <jcking at google.com>

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/11835/files
  - new: https://git.openjdk.org/jdk/pull/11835/files/04d84346..a86a30c4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11835&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11835&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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