Remove unused depends list from getblocktemplate transactions

Summary:
Prior to CTOR, each transaction provided with getblocktemplate included a numerical list
of prior transcations (by index) that the given transaction depended on. (See: BIP022)

However, once CTOR was activated, this list became meaningless as there is no longer a
topological ordering contraint. Worse, since dependent transactions can appear later
in a block, this data does not indicate reliably to clients if there are dependencies.

Finally, the specification allows this value to be omitted. (See BIP022)

This commit, therefore, removes the extraneous and erroreous calculation.

Test Plan: make check && ./test/functional/test_runner.py

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

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2782