Remove regexpengine workaround

[Shougo]

I have removed regexpengine workaround. Because, regexpengine option is global option.

It should not be changed by plugin.

                        *'regexpengine'* *'re'*
'regexpengine' 're' number  (default 0)
            global
    This selects the default regexp engine. |two-engines|
    The possible values are:
        0   automatic selection
        1   old engine
        2   NFA engine
    Note that when using the NFA engine and the pattern contains something
    that is not supported the pattern will not match.  This is only useful
    for debugging the regexp engine.
    Using automatic selection enables Vim to switch the engine, if the
    default engine becomes too costly.  E.g., when the NFA engine uses too
    many states.  This should prevent Vim from hanging on a combination of
    a complex pattern with long text.

[Shougo]

For example, if the plugin change the value, yats.vim will be very slow. https://github.com/HerringtonDarkholme/yats.vim/issues/135

[Shougo]

Thank you!