sh4: remove architecture

Support for SuperH was dropped over a decade ago. There no longer is any
architecture-specific code to be found, so just remove the corresponding
test. Technically it is still possible to compile FFmpeg as the
"generic" (pure C) architecture.

configure

@@ -2146,7 +2146,6 @@ ARCH_LIST="
     ppc64
     riscv
     s390
-    sh4
     sparc
     sparc64
     tilegx
@@ -5249,9 +5248,6 @@ case "$arch" in
     s390|s390x)
         arch="s390"
     ;;
-    sh4|sh)
-        arch="sh4"
-    ;;
     sun4*|sparc*)
         arch="sparc"
     ;;

libavcodec/sh4/README

@@ -1,6 +0,0 @@
-SH4 optimizations have been removed in
-commit d6096a67422534918405abb46dafbbac4608cbc3
-The last revission with the optimizations is cbfc9046e1c7e295b74f252902ae6f255eef4e78
-
-If you want to maintain these (or other) SH4 optimizations in ffmpeg, then please
-contact ffmpeg-devel@ffmpeg.org