[secp256k1] remove guava dep

Summary: remove overkill depencency to guava

Test Plan:
execute previous tests successfully

the JNI part can be built and tested by simply doing

$ ./autogen.sh
$ ./configure --enable-jni --enable-experimental --enable-module-ecdh
$ make check-java

Once https://reviews.bitcoinabc.org/D2686 is passed we can do this automatically

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien

Subscribers: Fabien, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2681