Move urlDecode to where it is used

Summary: It has just one call site and is creating circular dependencies betweent he wallet and server libraries.

Test Plan:
With and without wallet:

make

Test cmake build:

cmake -GNinja ..
ninja

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1022