RFR (trivial): 8216189: Remove Klass::compute_is_subtype_of

[Claes Redestad]

Hi,

Klass::compute_is_subtype_of appears to be effectively unused
and could be removed.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8216189
Webrev: http://cr.openjdk.java.net/~redestad/8216189/open.00/

Testing: tier1

/Claes