Makefile: release: really drop build/docs from release tarball

Commit 15cb9876 (release: remove manual build files from release
tarballs) tried to remove the temporary files from the manual build from the
release tarball, but manual-clean only removes build/docs/manual and leaves
build/docs in the tarball.

Instead use 'make clean' to completely remove the build directory from the
tarball.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>