tests: schema: Remove useless perf schema data

We have a test case that excercises the parser and formatter now which
takes part in schema checking so remove the schema-only test.

tests/domainschemadata/domain-perf-simple.xml

-<domain type='qemu'>
-  <name>QEMUGuest1</name>
-  <uuid>c7a5fdbd-edaf-9455-926a-d65c16db1809</uuid>
-  <currentMemory unit='KiB'>219136</currentMemory>
-  <vcpu placement='static'>1</vcpu>
-  <os>
-    <type arch='x86_64' machine='pc'>hvm</type>
-    <boot dev='hd'/>
-  </os>
-  <perf>
-      <event name='cmt' enabled='yes'/>
-  </perf>
-  <clock offset='utc'/>
-  <on_poweroff>destroy</on_poweroff>
-  <on_reboot>restart</on_reboot>
-  <on_crash>destroy</on_crash>
-  <devices>
-    <emulator>/usr/bin/qemu</emulator>
-  </devices>
-</domain>