Remove ParserTestFiles from extension.json
Remove ParserTestFiles from extension.json

Since it's already in tests/parser/, it will be autodiscovered by
the test runner.

Bug: T192234
Change-Id: Ie2c5737146c1df89a64ab4b679e47e48c590b4ca

wikimedia/mediawiki-extensions-Variables,wikimedia/mediawiki-extensions-Variables