Remove unused json_request()

Since I404e43e2e6ffecb5131cdcc43df03b8da0e7f0f8 which has added
json_request(), this method has never been used at all.
So this patch removes the method before migrating glance_http
module to tempest-lib.

Partially implements blueprint consistent-service-method-names

Change-Id: If20a33ea6adcdf91c4440f3efbf02b1388de9dbf
2 files changed