RFR: 8254776: Remove unimplemented LowMemoryDetector::check_memory_usage

[Aleksey Shipilev]

`LowMemoryDetector::check_memory_usage` seems declared but not implemented. Current history does not show any
definitions since the initial load. Can be removed.

Testing:
 - [x] Linux x86_64 build
 - [x] Test search for `check_memory_usage` in `src/hotspot`

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

Commit messages:
 - 8254776: Remove unimplemented LowMemoryDetector::check_memory_usage

Changes: https://git.openjdk.java.net/jdk/pull/659/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=659&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254776
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/659.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/659/head:pull/659

PR: https://git.openjdk.java.net/jdk/pull/659