[ImportVerilog] Remove unused variable, NFC

Remove an unused location variable that was tracking the location of a
substitution variable.  This isn't needed for the kinds of errors that can
be emitted by this code.  (This variable wasn't supposed to be used in
place of `loc`.)

Signed-off-by: Schuyler Eldridge <[email protected]>

Author: seldridge

lib/Conversion/ImportVerilog/FormatStrings.cpp

@@ -113,7 +113,6 @@ struct FormatStringParser {
     assert(!arguments.empty() && "Slang guarantees correct arg count");
     const auto &arg = *arguments[0];
     arguments = arguments.drop_front();
-    auto argLoc = context.convertLocation(arg.sourceRange);
 
     // Handle the different formatting options.
     // See IEEE 1800-2017 § 21.2.1.2 "Format specifications".