Revert "binman: btool: gzip: fix packer name so that binary can be found"

This reverts commit daa2da75.

This commit is not needed anymore since the btool_ prefix is
automatically stripped by bintool.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>