remove an unneeded check

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    |no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

It is already checked in the `isEnabled()` method if the current PHP
build is HHVM and the `server:run` command is never enabled then.
Thus, it's not needed to check for HHVM on every command execution.