Markdown optimization (follow-up)

The CL in [1] added a Markdown back-reference to documentation
sections, but appended the suffix string unconditionally, even
when not generting Markdown output. This polluted the command-line
help, for example:

```
$ gn help clean
gn clean <out_dir>...&nbsp;[Back to Top](#gn-reference)

  Deletes the contents of the output directory except for args.gn and
  creates a Ninja build environment sufficient to regenerate the build.
```

This CL fixes the situation by adding an `if (is_markdown)` test
to cleanup the output.

[1] https://gn-review.googlesource.com/c/gn/+/17420

Change-Id: Idd3fb6bd31caea84c7a105af334ae1eb69aae385
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/17560
Reviewed-by: Takuto Ikuta <tikuta@google.com>
Commit-Queue: David Turner <digit@google.com>

src/gn/standard_out.cc

diff --git a/src/gn/standard_out.cc b/src/gn/standard_out.cc
index 2ff353c..85cd665 100644
--- a/src/gn/standard_out.cc
+++ b/src/gn/standard_out.cc
@@ -310,7 +310,9 @@
       OutputString(line.substr(0, chars_to_highlight), DECORATION_YELLOW);
       OutputString(line.substr(chars_to_highlight));
       if (first_header) {
-        OutputString(" [Back to Top](#gn-reference)", DECORATION_NONE);
+        if (is_markdown) {
+          OutputString(" [Back to Top](#gn-reference)", DECORATION_NONE);
+        }
         first_header = false;
       }
       OutputString("\n");