[SECP256K1] Travis Don't run the constant time check with java tests

Summary:
The autotools check-java target doesn't build the
valgrind_ctime_test, causing Travis to fail.
Since the case (without JNI) it already covered, it is safe to disable
the check for this case.

Test Plan:
Run the Travis build (see
https://travis-ci.org/github/Fabcien/secp256k1/builds/668626198).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5598