Remove unused string.

We saved the string in the lexer try_float for use in error messages,
then changed the message to not show the string. This CL removes the
creation of the float string.

Change-Id: I9bb381bebe31f25609c66d5ccda91de0277199cc
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/97069
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Auto-Submit: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/tint/reader/wgsl/lexer.cc

@@ -357,9 +357,6 @@ Token Lexer::try_float() {
         return {};
     }
 
-    // Save the error string, for use by diagnostics.
-    const auto str = std::string{substr(start, end - start)};
-
     advance(end - start);
     end_source(source);