Drop build-api support

It appears the build-api effort at
https://github.com/cgwalters/build-api hasn't really caught on.
systemd appears one of the very few projects actually supporting it.

It does confuse certain tools though. E.g. debhelper by finding a
configure script wrongly assumes this is an autoconf project and thus
needs to be told explicitly that this is in fact a Meson project [1].

Given that Meson is an established build system by now, it appears ok to
drop this compat layer, which will never be fully complete anyway.

[1] https://salsa.debian.org/systemd-team/systemd/-/blob/debian/master/debian/rules?ref_type=heads#L281

Makefile

@@ -1,7 +0,0 @@
-# SPDX-License-Identifier: LGPL-2.1-or-later
-
-all:
-	ninja -C build
-
-install:
-	DESTDIR=$(DESTDIR) ninja -C build install

configure

@@ -1,24 +0,0 @@
-#!/usr/bin/env bash
-# SPDX-License-Identifier: LGPL-2.1-or-later
-set -e
-
-cflags="CFLAGS=${CFLAGS-}"
-cxxflags="CXXFLAGS=${CXXFLAGS-}"
-args=()
-
-for arg in "$@"; do
-    case "$arg" in
-        CFLAGS=*)
-            cflags="$arg"
-            ;;
-        CXXFLAGS=*)
-            cxxflags="$arg"
-            ;;
-        *)
-            args+=("$arg")
-    esac
-done
-
-export "${cflags?}" "${cxxflags?}"
-set -x
-exec meson setup build "${args[@]}"