Remove unused variable in statext_mcv_serialize()

Remove unused variable in statext_mcv_serialize()

The itemlen variable used to be referenced in multiple places, but since
reworking the serialization code it's used only in one assert. Fixed by
removing the variable and calling the macro from the assert directly.

Backpatch to 12, where this code was introduced.

Reported-by: Jeff Janes
Discussion: https://postgr.es/m/CAMkU=1zc_ovH9NZd_9ovuiEWkF9yX06URUDdXCmgDydf-bqB5A@mail.gmail.com