build: Delete Makefile (#2641)

The `Makefile` was added in
53a4f3b,
and at that time, it was called from CI. Since then, our CI no longer
calls `make` to build, lint, and test Sentry CLI, so the `Makefile` no
longer appears to serve much of a purpose.

So, let's delete the file to avoid confusing anyone as to its purpose.

Author: szokeasaurusrex