capstone: Remove the capstone submodule

Now that we allow compiling with Capstone v3.0.5 again, all our supported
build hosts should provide at least this version of the disassembler
library, so we do not need to ship this as a submodule anymore.

Message-Id: <20220516145823.148450-4-thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>