rustdoc: remove outdated CSS .content table etc

The `.content table` / `.content td` / `.content tr` family of selectors date
back to 4fd061c, when module indexes and
other parts of rustdoc used `<table>` tags for layout and content
presentation. The `.content td h1, .content td h2` has only been changed
since then to tweak the font size in
dd5ff42.

https://github.com/rust-lang/rust/blob/4fd061c426902b0904c65e64a3780b21f9ab3afb/src/rustdoc_ng/html/static/main.css#L155-L162

This CSS would have affected:

  * search result tables, which were removed in
    b615c0c
  * module item tables, which were removed in
    6020c79
  * docblock tables from markdown, which still exist

It may also have affected a few other tables over the last decade, but
they've been gradually replaced with grid layouts and flexbox to make layouts
that work better on narrow viewports. For example,
34bd2b8.

These rules have no affect on the appearance of docblock tables
===============================================================

    .content table {
        border-spacing: 0 5px;
    }

According to MDN, [border-spacing] only has an effect when `border-collapse`
is `separate`. However, `border-collapse: collapse` is set globally for all
tables, so this rule does nothing.

[border-spacing]: https://developer.mozilla.org/en-US/docs/Web/CSS/border-spacing

    .content td p:first-child { margin-top: 0; }

Tables with paragraphs in them are impossible without dropping down to raw
HTML. Also, the rustdoc stylesheet sets paragraphs to have no top margin
anyway, so this rule is a no-op.

    .content td h1, .content td h2 { margin-left: 0; font-size: 1.125rem; }

Tables with headers in them are impossible without dropping down to raw HTML.
This is considered unlikely, especially since it looks weird right now
(`.docblock h2` has an underline that is redundant with the table cell's own
border).

    .content tr:first-child td { border-top: 0; }

This has no effect because of border collapsing.

This rule is removed, because tables look fine without it
=========================================================

    .content td:first-child { padding-right: 20px; }

By removing this rule, the first cell in each row has the same padding as all
other cells in the row.

This rule is kept, and converted to directly target `.docblock`
===============================================================

    .content td { vertical-align: top; }

Removing this rule would cause it to be aligned to the middle instead.

src/librustdoc/html/static/css/rustdoc.css

@@ -686,15 +686,6 @@ pre, .rustdoc.source .example-wrap {
 	position: relative;
 }
 
-.content table {
-	border-spacing: 0 5px;
-}
-.content td { vertical-align: top; }
-.content td:first-child { padding-right: 20px; }
-.content td p:first-child { margin-top: 0; }
-.content td h1, .content td h2 { margin-left: 0; font-size: 1.125rem; }
-.content tr:first-child td { border-top: 0; }
-
 .docblock table {
 	margin: .5em 0;
 	width: calc(100% - 2px);
@@ -705,6 +696,7 @@ pre, .rustdoc.source .example-wrap {
 .docblock table td {
 	padding: .5em;
 	border: 1px dashed var(--border-color);
+	vertical-align: top;
 }
 
 .docblock table th {