Clean up param handling in ApiStashEdit

Clean up param handling in ApiStashEdit

It doesn't make sense to submit both stashedtexthash and text, so
requireOnlyOneParameter() is correct.  This simplifies the code and
gives a more helpful error message. (Previously if both parameters were
passed, we would ignore text unless stashedtexthash was empty, in which
case we would ignore it.)

Change-Id: I306b15eefb6fd4379a3eed88d84113c2e43c4a95