doc: remove darwin/386, darwin/arm from ports list

Updates #37611.

Change-Id: I7ae5a61d2e2189fd48ac3548e370e0de1dd79832
Reviewed-on: https://go-review.googlesource.com/c/go/+/227343
Run-TryBot: Austin Clements <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Cherry Zhang <[email protected]>

Author: aclements

doc/install-source.html

@@ -540,15 +540,9 @@ <h2 id="environment">Optional environment variables</h2>
 <td></td><td><code>android</code></td> <td><code>arm64</code></td>
 </tr>
 <tr>
-<td></td><td><code>darwin</code></td> <td><code>386</code></td>
-</tr>
-<tr>
 <td></td><td><code>darwin</code></td> <td><code>amd64</code></td>
 </tr>
 <tr>
-<td></td><td><code>darwin</code></td> <td><code>arm</code></td>
-</tr>
-<tr>
 <td></td><td><code>darwin</code></td> <td><code>arm64</code></td>
 </tr>
 <tr>