RFR: 8283246: WebKit fails to build on Linux

[Peter Zhelezniakov]

The file `wtf/linux/MemoryPressureLinux.cpp` was renamed `wtf/unix/MemoryPressureUnix.cpp` in https://trac.webkit.org/changeset/261428 . In the openjfx workspace, both files are present. I'm removing the old file.

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Commit messages:
 - 8283246: WebKit fails to build on Linux

Changes: https://git.openjdk.java.net/jfx/pull/756/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=756&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283246
  Stats: 142 lines in 2 files changed: 1 ins; 141 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/756.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/756/head:pull/756

PR: https://git.openjdk.java.net/jfx/pull/756