RFR: JDK-8081694 Remove DISABLED_WARNINGS_gcc for libsctp

[Magnus Ihse Bursie]

When fixing JDK-8081616 
<https://bugs.openjdk.java.net/browse/JDK-8081616>, it turned out that
DISABLED_WARNINGS_gcc := unused-parameter
seemed to be needed for libsctp in make/lib/Lib-jdk.sctp.gmk.

This is not be needed anymore (and possibly never was).

Bug: https://bugs.openjdk.java.net/browse/JDK-8081694
Patch inline:
diff --git a/make/lib/Lib-jdk.sctp.gmk b/make/lib/Lib-jdk.sctp.gmk
--- a/make/lib/Lib-jdk.sctp.gmk
+++ b/make/lib/Lib-jdk.sctp.gmk
@@ -45,7 +45,6 @@
              $(LIBJAVA_HEADER_FLAGS) \
              -I$(SUPPORT_OUTPUTDIR)/headers/jdk.sctp \
              -I$(SUPPORT_OUTPUTDIR)/headers/java.base, \
-        DISABLED_WARNINGS_gcc := unused-parameter, \
          MAPFILE := $(JDK_TOPDIR)/make/mapfiles/libsctp/mapfile-vers, \
          LDFLAGS := $(LDFLAGS_JDKLIB) \
              $(call SET_SHARED_LIBRARY_ORIGIN), \