Eliminate forked FunctionCallSignature in favor of upstream version

Per @gsherwood's recommendation, see https://github.com/squizlabs/PHP_CodeSniffer/issues/318

We need to determine if the customizations done to the sniff will allow us to abandon our fork and use upstream instead.

Some things to look out for when looking at the history of the sniff:
 * 8bdbcbcf1259d08ff0128d648f648d8f1ab545a0: Allow for tab indention in multi line function calls
 * b1bf321aec34278e8fa7fb4d92dac6ba74a68d8: Switch multiline function call parenthesis check to opt-in (#41)
 * f8b7a12471d0170da4c062948b257d0ba22cd9be: Extend to include other 'functions'. Extend sniff and unit tests to include ‘function’ language constructs like empty() and isset()
 * fd766a58d6bd0d92e5b3b580bf537b2a3ea196cb: Allow blank lines in multiline function definitions (#249)