Editorial: Remove If in SystemTimeZoneIdentifier (tc39#3104)

Removes an unnecessary line in SystemTimeZoneIdentifier.

Originally this AO contained an assert that applied to named time zones
but not offset time zones. Based on editors' feedback in tc39#3035, this
assertion was replaced with different text, making the remaining If
statement unnecessary. But I didn't notice this until after tc39#3035 was
merged.

This commit removes the now-unnecessary If statement.

spec.html

@@ -32343,7 +32343,6 @@ <h1>SystemTimeZoneIdentifier ( ): a String</h1>
         <emu-alg>
           1. If the implementation only supports the UTC time zone, return *"UTC"*.
           1. Let _systemTimeZoneString_ be the String representing the host environment's current time zone, either a primary time zone identifier or an offset time zone identifier.
-          1. If IsTimeZoneOffsetString(_systemTimeZoneString_) is *true*, return _systemTimeZoneString_.
           1. Return _systemTimeZoneString_.
         </emu-alg>