Remove Html source mappings from the compiler (#12003)

I added Html source mappings to the compiler a little while ago, in
order to alter how Html documents are produced, to try to improve the
story around diagnostics, specifically with JavaScript and CSS language
services having trouble interpreting the `~`s we produce in place of C#
code in Html documents. The theory was also to improve some formatting
quirks, but the old formatting engine proved such a pain to update to
support source mappings, or any type of alternate Html document
generation, that we never actually used the feature in tooling.

In hindsight, I don't think we'll ever be able to actual use the system,
so it seems pretty wasteful to have the compiler produce this extra
data, and to process it for no reason in tooling. The reason we won't be
able to use it is because we fundamentally don't understand what we'd
need to output in place of the C# anyway, and since we've moved to
outputting `/*~~*/` comments in place of long runs of C#, the number of
issues we see from the system has dropped dramatically.

Essentially, given something like:

```
<style>
    body {
        font-size: @fontsize
    }
</style>

<script>
    [email protected] = "Font size is " + @fontsize
    alert(@message);
</script>
```

In order to produce "better" Html documents, we'd have to understand,
for each piece of C#, the underlying context in either Html, Css or
JavaScript, in order to output something that compiles. If we can't do
that, outputting comments as we currently do is just as good, so the
only thing Html source mappings would buy us is the ability to not have
to match 1:1 document coordinates with Razor, but it doesn't seem to me
like that cost is very high.

Feel free to disagree if you think this system is worth keeping in place
for the future, I'm happy to discuss.

Oh, commit-at-a-time is probably best, otherwise GitHub might complain.

Author: davidwengier

src/Compiler/Microsoft.AspNetCore.Razor.Language/test/IntegrationTests/CodeGenerationIntegrationTest.cs

@@ -347,7 +347,6 @@ private void DesignTimeTest(string testName)
         AssertHtmlDocumentMatchesBaseline(codeDocument.GetHtmlDocument(), testName);
         AssertCSharpDocumentMatchesBaseline(codeDocument.GetRequiredCSharpDocument(), testName);
         AssertSourceMappingsMatchBaseline(codeDocument, testName);
-        AssertHtmlSourceMappingsMatchBaseline(codeDocument, testName);
         AssertLinePragmas(codeDocument);
         AssertCSharpDiagnosticsMatchBaseline(codeDocument, testName);
     }
@@ -417,7 +416,6 @@ private void RunDesignTimeTagHelpersTest(IEnumerable<TagHelperDescriptor> descri
         AssertDocumentNodeMatchesBaseline(codeDocument.GetRequiredDocumentNode(), testName);
         AssertCSharpDocumentMatchesBaseline(codeDocument.GetRequiredCSharpDocument(), testName);
         AssertHtmlDocumentMatchesBaseline(codeDocument.GetHtmlDocument(), testName);
-        AssertHtmlSourceMappingsMatchBaseline(codeDocument, testName);
         AssertSourceMappingsMatchBaseline(codeDocument, testName);
         AssertCSharpDiagnosticsMatchBaseline(codeDocument, testName);
     }