Drop build support for LEGACY_USE_JAVA7.

This flag allowed for building with an OpenJDK 7 toolchain. It was
used for build bot builds that now work with an OpenJDK 8 toolchain.
Hence, this feature is no longer required. This CL drops it.

Bug: 27583810
Test: Treehugger build succeeds.
Change-Id: I7efca363d665292bfb88f4a4d6f3a120fc8f93f3
diff --git a/core/droiddoc.mk b/core/droiddoc.mk
index a70ab03..3438757 100644
--- a/core/droiddoc.mk
+++ b/core/droiddoc.mk
@@ -215,7 +215,7 @@
                 \@$(PRIVATE_SRC_LIST_FILE) \
                 -J-Xmx1024m \
                 -XDignore.symbol.file \
-                $(if $(LEGACY_USE_JAVA7),,-Xdoclint:none) \
+                -Xdoclint:none \
                 $(PRIVATE_PROFILING_OPTIONS) \
                 $(addprefix -classpath ,$(PRIVATE_CLASSPATH)) \
                 $(addprefix -bootclasspath ,$(PRIVATE_BOOTCLASSPATH)) \