mesa: Use a sed instead of patch for LLVM-17 fix

When the patch can be converted to a not-so-long sed, we prefer the sed
because it tells people "what this command is doing" more explicitly and
also reduces an additional download.  And for patch or sed we need a
<para> describing "what it fixes".

It's also a bad idea to fold the patch/sed command just before "meson"
in the same <screen> because it'd be too easy to overread it.

x/installing/mesa.xml

@@ -75,14 +75,6 @@
 
     <bridgehead renderas="sect3">Additional Downloads</bridgehead>
     <itemizedlist spacing="compact">
-    <!-- Remove this once mesa reaches 23.2 because it is already merged in the new branch-->
-      <listitem>
-        <para>
-          Required patch:
-          <ulink url="&patch-root;/mesa-23.1.8-llvm17-fix-1.patch"/>
-          Fixes a build issue relating to a removed header in llvm-17
-        </para>
-      </listitem>
     <!-- 9 Apr 23.  Change to unversioned patch since it never changes. -->
       <listitem>
         <para>
@@ -275,13 +267,21 @@
 DRI_DRIVERS="i965,nouveau"</userinput></screen>
 -->
 
+    <!-- https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22021 -->
+    <para>
+      Remove two unneeded references to LLVM headers to avoid a build
+      failure with LLVM 17 or later:
+    </para>
+
+<screen><userinput>sed '/Scalar.h/d;/Utils.h/d' \
+    -i src/gallium/auxiliary/gallivm/lp_bld_init.c</userinput></screen>
+
     <para>
       Install <application>Mesa</application> by running the following
       commands:
     </para>
 
-<screen><userinput>patch -Np1 -i ../mesa-23.1.8-llvm17-fix-1.patch
-mkdir build &amp;&amp;
+<screen><userinput>mkdir build &amp;&amp;
 cd    build &amp;&amp;
 
 meson setup                   \