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

Commit 967c885108 "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 ccadd6bcba "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>

tests/unit/test-qmp-cmds.c

@@ -13,13 +13,6 @@
 
 static QmpCommandList qmp_commands;
 
-#if defined(TEST_IF_STRUCT) && defined(TEST_IF_CMD)
-UserDefThree *qmp_TestIfCmd(TestIfStruct *foo, Error **errp)
-{
-    return NULL;
-}
-#endif
-
 UserDefThree *qmp_TestCmdReturnDefThree(Error **errp)
 {
     return NULL;