remove unused variables in JSON-parsing code

[jameslamb]

Proposes removing two unused variables in src/common/json.cc. Hopefully this will cut a bit of processing time and a some unnecessary allocations out of JSON-reading operations.

Notes for Reviewers

I found these by running cppcheck over the project's source code.

cppcheck \
    --force \
    --enable=all \
    --std=c++14 \
    -I include/ \
    -UDEBUG \
    src/ \
> cppcheck.txt 2>&1

cat cppcheck.txt | grep 'unusedVar'

These are the only two such warnings cppcheck found.

src/common/json.cc:440:22: style: Unused variable: output [unusedVariable]
src/common/json.cc:659:15: style: Unused variable: buffer [unusedVariable]

[trivialfis]

Apologies for the slow response and thank you for the cleanup!