RFR: 8146871: Make the clean target silent in hotspot/test/Makefile

[Erik Helin]

Hi all,

this very small patches silences the 'clean' target in
hotspot/test/Makefile (the 'prep' target is already silent).

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8146871

Webrev:
http://cr.openjdk.java.net/~ehelin/8146871/00/webrev/

Testing:
- Buiding locally and verified that the commands are no longer listed
- JPRT

Thanks,
Erik