RFR: JDK-8215304 Make target "docs-jdk-index" has unnecessary dependencies

[Magnus Ihse Bursie]

The target  docs-jdk-index has dependencies that are quite heavyweight, 
and are not needed. They should just be removed.

Bug: https://bugs.openjdk.java.net/browse/JDK-8215304
Patch inline:
diff --git a/make/Main.gmk b/make/Main.gmk
--- a/make/Main.gmk
+++ b/make/Main.gmk
@@ -862,8 +862,6 @@
    docs-jdk-specs: $(JVM_DOCS_TARGETS) jdk.jdi-gensrc \
        docs-jdk-index

-  docs-jdk-index: exploded-image buildtools-modules
-
    docs-zip: docs-jdk

    # Tests

/Magnus