removed unneeded check

It is already checked in the `isEnabled()` method of the parent
`ServerCommand` class if the current PHP build is HHVM and the
`server:start` command is never enabled then. Thus, it's not needed
to check for HHVM on every command execution.

This was pointed out by @stof in #12489 for the `server:run` command.
minor #12552 [FrameworkBundle] removed unneeded check (xabbuh)

This PR was merged into the 2.6 branch.

Discussion
----------

[FrameworkBundle] removed 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 of the parent
`ServerCommand` class if the current PHP build is HHVM and the
`server:start` command is never enabled then. Thus, it's not needed
to check for HHVM on every command execution.

This was pointed out by @stof in #12489 for the `server:run` command.

Commits
-------

ce2adfa removed unneeded check