RFR: 8261109: [macOS] Remove disabled warning for JNF in make/autoconf/flags-cflags.m4

[Phil Race]

remove un-needed disabling now JNF has gone ..

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Commit messages:
 - 8261109: [macOS] Remove disabled warning for JNF in make/autoconf/flags-cflags.m4

Changes: https://git.openjdk.java.net/jdk/pull/2396/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2396&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261109
  Stats: 5 lines in 1 file changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2396.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2396/head:pull/2396

PR: https://git.openjdk.java.net/jdk/pull/2396