Remove `./configure` and require `autoconf` for building Citus

**Proposal**:
- Require that everyone who builds Citus has `autoconf` installed, so they're able to build Citus
- Remove `./configure` from the repository

**Benefits**

This will make diffs smaller and more meaningful when they include changes to `./configure.in`. It also reduces the (admittedly already small) chance of merge conflicts if multiple branches edit `./configure`.

**Rationale**

Different versions of `autoconf` emit different `./configure` files, so we have large diffs whenever `./configure.in` changes, depending on who makes the change.

@byucesoy [made a valiant effort](https://github.com/citusdata/citus/pull/1769) at solving the problem but I think there's an easier solution than trying to convince all autoconf versions to emit the same file.

Some notes:
- This doesn't involve messing with our Makefile. Our Makefile already generates `./configure` when `configure.in` changes, so removing `./configure` is all this change takes.