billing tests: Remove unnecessary orjson.dumps of params.
billing tests: Remove unnecessary orjson.dumps of params.

This should have been in https://github.com/zulip/zulip/pull/18066.

The reason, the tests were not failing inspite of the params being
json encoded was because pretty much all these tests did not test
the functionality of the endpoint. Rather they were testing things
like whether the user has the right to access the endpoint and all.
So the value of the params did not matter.

The only one test which is an exception is test_replace_payment_source.
Even though a json encoded token was passed to an endpoint that
expecteda string, the test continued to work becausethe fixtures were
not updated for the test in that PR, so instead of sending an incorrect
json encoded token to stripe endpoint it was sending the correct string
token. Now that we removed the json.dumps of token, we no longer have to
update the fixtures.

I have run the tests with --generate-stripe-fixtures set to True and all
the tests are passing. Not including the fixture changes since the tests
conntinue to work the same with both the existing and new fixtures.

rht/zulip,zulip/zulip,zulip/zulip,zulip/zulip,punchagan/zulip,andersk/zulip,rht/zulip,kou/zulip,punchagan/zulip,eeshangarg/zulip,punchagan/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,hackerkid/zulip,kou/zulip,kou/zulip,punchagan/zulip,punchagan/zulip,hackerkid/zulip,eeshangarg/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,zulip/zulip,rht/zulip,hackerkid/zulip,andersk/zulip,hackerkid/zulip,punchagan/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip,hackerkid/zulip,rht/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,kou/zulip,punchagan/zulip