JDK-8187542 Remove superfluous *_TOPDIR variables

[Magnus Ihse Bursie]

After the forest consolidation, the makefiles contains a number of 
<component>_TOPDIR variables, that are no longer used. They should be 
cleaned out.

The intention is to push this to the consolidated forest, once it opens.
Bug: https://bugs.openjdk.java.net/browse/JDK-8187542
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8187542-remove-extra-TOPDIR-variables/webrev.01

/Magnus