Remove explicit reference to the Squiz.WhiteSpace.SuperfluousWhitespace sniff

[aadmathijssen]

This PR removes the explicit inclusion of the Squiz.WhiteSpace.SuperfluousWhitespace sniff.

This is unnecessary, as this sniff is already included via the PSR12 ruleset:

    <!-- There MUST NOT be trailing whitespace at the end of lines.
    Blank lines MAY be added to improve readability and to indicate related blocks of code except where explicitly forbidden. -->
    <rule ref="Squiz.WhiteSpace.SuperfluousWhitespace"/>
    <rule ref="Squiz.WhiteSpace.SuperfluousWhitespace.StartFile">
        <severity>0</severity>
    </rule>
    <rule ref="Squiz.WhiteSpace.SuperfluousWhitespace.EndFile">
        <severity>0</severity>
    </rule>
    <rule ref="Squiz.WhiteSpace.SuperfluousWhitespace.EmptyLines">
        <severity>0</severity>
    </rule>

Note that the ISAAC ruleset explicitly specified a value for the ignoreBlankLines property, whereas the PSR12 ruleset does not. However, this doesn't make a difference, since the false value is the default value of that property.