Remove a workaround in avm.cmd

The bug has been fixed in avm research-v6.0.0.

ext/avm.cmd

@@ -12,11 +12,6 @@ git clone -b research-v6.0.0 --depth 1 https://gitlab.com/AOMediaCodec/avm.git
 
 cd avm
 
-: # The following fix avoids errors such as "libaom.a: in function `write_frame_hash': bitstream.c: undefined reference to `MD5Init'"
-: # TODO(yguyon): Remove at next version bump. It was fixed in https://gitlab.com/AOMediaCodec/avm/-/merge_requests/752.
-sed 's-"${AOM_ROOT}/av1/encoder/dwt.h"-"${AOM_ROOT}/av1/encoder/dwt.h"\n  "${AOM_ROOT}/common/md5_utils.c"\n  "${AOM_ROOT}/common/md5_utils.h"\n  "${AOM_ROOT}/common/rawenc.c"\n  "${AOM_ROOT}/common/rawenc.h"-g' av1/av1.cmake > av1/av1.cmake.sed
-mv av1/av1.cmake.sed av1/av1.cmake
-
 mkdir build.libavif
 cd build.libavif