sh: sh3: Remove CPU support

This CPU core is old, no boards using the CPU are left in mainline,
it has no prospects of ever being converted to DM, drop it.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Chris Brandt <chris.brandt@renesas.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Cc: Vladimir Zapolskiy <vz@mleia.com>
Cc: Yoshihiro Shimoda <shimoda.yoshihiro.uh@renesas.com>
diff --git a/doc/README.sh b/doc/README.sh
index 667c797..766a8c8 100644
--- a/doc/README.sh
+++ b/doc/README.sh
@@ -23,13 +23,7 @@
 	2.2. Renesas SH7722
 		This CPU has the SH4AL-DSP core.
 
-	2.3. Renesas SH7720
-		This CPU has the SH3 core.
-
-	2.4. Renesas SH7710/SH7712
-		This CPU has the SH3-DSP core and Ethernet controller.
-
-	2.5. Renesas SH7780
+	2.3. Renesas SH7780
 		This CPU has the SH4A core.
 
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