serializer: Remove JSON indentation and dict key sorting

Serializing to JSON using “simplejson” is significantly slower when
indentation and/or sorting of dictionary keys is used. In simplejson 1.x
the difference isn't that big, but with simplejson 2.x the difference
can be up to a factor of 7.5. The reason is that the latter no longer
uses C functions when sorting or indentation is used.

With this patch we revert everything to simplejson's defaults, which
should provide us with the best performance available.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>