Implement delete function for the RadixTree

Summary:
It is failry similar to an insert operation, except it never generates a subtree.

Return false if the key is not found in the tree, true if it is.

Test Plan: Added test cases.

Reviewers: #bitcoin_abc, jasonbcox, Fabien

Reviewed By: #bitcoin_abc, jasonbcox, Fabien

Subscribers: jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2336