tests-qmp-cmds: Drop unused and incorrect qmp_TestIfCmd()

Commit 967c8851 "qapi: add 'if' to top-level expressions" added
command TestIfCmd with an 'if' condition.  It also added the
qmp_TestIfCmd() to go with it, guarded by the corresponding #if.
Commit ccadd6bc "qapi: Add 'if' to implicit struct members" changed
the command, but not the function.  Compiles only because we don't
satisfy the #if.  Instead of fixing the function, simply drop it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210323094025.3569441-22-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>