Remove public error properties of sniffs that don't need them

[jrfnl]

Description

Fixes squizlabs/PHP_CodeSniffer#2823

Related to #186

Suggested changelog entry

Removed:

• Removed error property of sniff Generic.Strings.UnnecessaryStringConcat
  • This sniff now always produces errors
  • To make this sniff produce warnings, include the following in a ruleset.xml or [.]phpcs.xml[.dist] file:

    <rule ref="Generic.Strings.UnnecessaryStringConcat">
        <type>warning</type>
    </rule>

• Removed error property of sniff Generic.Formatting.MultipleStatementAlignment.
  • This sniff now always produces warnings
  • The Generic.Formatting.MultipleStatementAlignment.IncorrectWarning error code has been removed.
    • Refer to the Generic.Formatting.MultipleStatementAlignment.Incorrect error code instead.
  • The Generic.Formatting.MultipleStatementAlignment.NotSameWarning error code has been removed.
    • Refer to the Generic.Formatting.MultipleStatementAlignment.NotSame error code instead.
  • To make this sniff produce errors, include the following in a ruleset.xml or [.]phpcs.xml[.dist] file:

    <rule ref="Generic.Formatting.MultipleStatementAlignment">
        <type>error</type>
    </rule>